Patchwork [2/2] target-i386: kvm: enable all supported KVM features for -cpu host

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Submitter Eduardo Habkost
Date Dec. 28, 2012, 6:37 p.m.
Message ID <1356719854-16401-3-git-send-email-ehabkost@redhat.com>
Download mbox | patch
Permalink /patch/208516/
State New
Headers show

Comments

Eduardo Habkost - Dec. 28, 2012, 6:37 p.m.
When using -cpu host, we don't need to use the kvm_default_features
variable, as the user is explicitly asking QEMU to enable all feature
supported by the host.

This changes the kvm_cpu_fill_host() code to use GET_SUPPORTED_CPUID to
initialize the kvm_features field, so we get all host KVM features
enabled.

This will also allow use to properly check/enforce KVM features inside
kvm_check_features_against_host() later. For example, we will be able to
make this:

  $ qemu-system-x86_64 -cpu ...,+kvm_pv_eoi,enforce

refuse to start if kvm_pv_eoi is not supported by the host (after we fix
kvm_check_features_against_host() to check KVM flags as well).

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
---
 target-i386/cpu.c | 2 ++
 1 file changed, 2 insertions(+)
Igor Mammedov - Jan. 2, 2013, 2:52 p.m.
On Fri, 28 Dec 2012 16:37:34 -0200
Eduardo Habkost <ehabkost@redhat.com> wrote:

> When using -cpu host, we don't need to use the kvm_default_features
> variable, as the user is explicitly asking QEMU to enable all feature
> supported by the host.
> 
> This changes the kvm_cpu_fill_host() code to use GET_SUPPORTED_CPUID to
> initialize the kvm_features field, so we get all host KVM features
> enabled.

1_2 and 1_3 compat machines diff on pv_eoi flag, with this patch 1_2 might
have it set.
Is it ok from compat machines pov?

> 
> This will also allow use to properly check/enforce KVM features inside
> kvm_check_features_against_host() later. For example, we will be able to
> make this:
> 
>   $ qemu-system-x86_64 -cpu ...,+kvm_pv_eoi,enforce
> 
> refuse to start if kvm_pv_eoi is not supported by the host (after we fix
> kvm_check_features_against_host() to check KVM flags as well).
It would be nice to have kvm_check_features_against_host() patch in this
series to verify that this patch and previous patch works as expected.

> 
> Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
> ---
>  target-i386/cpu.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/target-i386/cpu.c b/target-i386/cpu.c
> index 6e2d32d..76f19f0 100644
> --- a/target-i386/cpu.c
> +++ b/target-i386/cpu.c
> @@ -900,6 +900,8 @@ static void kvm_cpu_fill_host(x86_def_t *x86_cpu_def)
>      /* Other KVM-specific feature fields: */
>      x86_cpu_def->svm_features =
>                  kvm_arch_get_supported_cpuid(s, 0x8000000A, 0, R_EDX);
> +    x86_cpu_def->kvm_features =
> +                kvm_arch_get_supported_cpuid(s, KVM_CPUID_FEATURES, 0,
> R_EAX); 
>  #endif /* CONFIG_KVM */
>  }
Eduardo Habkost - Jan. 2, 2013, 3:29 p.m.
On Wed, Jan 02, 2013 at 03:52:45PM +0100, Igor Mammedov wrote:
> On Fri, 28 Dec 2012 16:37:34 -0200
> Eduardo Habkost <ehabkost@redhat.com> wrote:
> 
> > When using -cpu host, we don't need to use the kvm_default_features
> > variable, as the user is explicitly asking QEMU to enable all feature
> > supported by the host.
> > 
> > This changes the kvm_cpu_fill_host() code to use GET_SUPPORTED_CPUID to
> > initialize the kvm_features field, so we get all host KVM features
> > enabled.
> 
> 1_2 and 1_3 compat machines diff on pv_eoi flag, with this patch 1_2 might
> have it set.
> Is it ok from compat machines pov?

-cpu host is completely dependent on host hardware and kernel version,
there are no compatibility expectations.

> 
> > 
> > This will also allow use to properly check/enforce KVM features inside
> > kvm_check_features_against_host() later. For example, we will be able to
> > make this:
> > 
> >   $ qemu-system-x86_64 -cpu ...,+kvm_pv_eoi,enforce
> > 
> > refuse to start if kvm_pv_eoi is not supported by the host (after we fix
> > kvm_check_features_against_host() to check KVM flags as well).
> It would be nice to have kvm_check_features_against_host() patch in this
> series to verify that this patch and previous patch works as expected.

The kvm_check_features_against_host() change would be a user-visible
behavior change, and I wanted to keep the changes minimal by now. (the
main reason I submitted this earlier is to make it easier to clean up
the init code for CPU subclasses)

I was planning to introduce those behavior changes only after
introducing the feature-word array, so the kvm_check_features_against_host()
code can become simpler and easier to review (instead of adding 4
additional items to the messy struct model_features_t array). But if you
think we can introduce those changes now, I will be happy to send a
series that changes that code as well.

> 
> > 
> > Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
> > ---
> >  target-i386/cpu.c | 2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/target-i386/cpu.c b/target-i386/cpu.c
> > index 6e2d32d..76f19f0 100644
> > --- a/target-i386/cpu.c
> > +++ b/target-i386/cpu.c
> > @@ -900,6 +900,8 @@ static void kvm_cpu_fill_host(x86_def_t *x86_cpu_def)
> >      /* Other KVM-specific feature fields: */
> >      x86_cpu_def->svm_features =
> >                  kvm_arch_get_supported_cpuid(s, 0x8000000A, 0, R_EDX);
> > +    x86_cpu_def->kvm_features =
> > +                kvm_arch_get_supported_cpuid(s, KVM_CPUID_FEATURES, 0,
> > R_EAX); 
> >  #endif /* CONFIG_KVM */
> >  }
>
Igor Mammedov - Jan. 2, 2013, 8:30 p.m.
On Wed, 2 Jan 2013 13:29:10 -0200
Eduardo Habkost <ehabkost@redhat.com> wrote:

> On Wed, Jan 02, 2013 at 03:52:45PM +0100, Igor Mammedov wrote:
> > On Fri, 28 Dec 2012 16:37:34 -0200
> > Eduardo Habkost <ehabkost@redhat.com> wrote:
> > 
> > > When using -cpu host, we don't need to use the kvm_default_features
> > > variable, as the user is explicitly asking QEMU to enable all feature
> > > supported by the host.
> > > 
> > > This changes the kvm_cpu_fill_host() code to use GET_SUPPORTED_CPUID to
> > > initialize the kvm_features field, so we get all host KVM features
> > > enabled.
> > 
> > 1_2 and 1_3 compat machines diff on pv_eoi flag, with this patch 1_2 might
> > have it set.
> > Is it ok from compat machines pov?
> 
> -cpu host is completely dependent on host hardware and kernel version,
> there are no compatibility expectations.

It's still kind of unpleasant surprise if on the same host
"qemu-1.3 -cpu host -machine pc-1.2" and "qemu-1.3+ -cpu host -machine pc-1.2"
would give different pv_eoi feature default, where pv-eoi should be
available after -machine pc-1.2 by default.

> 
> > 
> > > 
> > > This will also allow use to properly check/enforce KVM features inside
> > > kvm_check_features_against_host() later. For example, we will be able to
> > > make this:
> > > 
> > >   $ qemu-system-x86_64 -cpu ...,+kvm_pv_eoi,enforce
> > > 
> > > refuse to start if kvm_pv_eoi is not supported by the host (after we fix
> > > kvm_check_features_against_host() to check KVM flags as well).
> > It would be nice to have kvm_check_features_against_host() patch in this
> > series to verify that this patch and previous patch works as expected.
> 
> The kvm_check_features_against_host() change would be a user-visible
> behavior change, and I wanted to keep the changes minimal by now. (the
> main reason I submitted this earlier is to make it easier to clean up
> the init code for CPU subclasses)
> 
> I was planning to introduce those behavior changes only after
> introducing the feature-word array, so the kvm_check_features_against_host()
> code can become simpler and easier to review (instead of adding 4
> additional items to the messy struct model_features_t array). But if you
> think we can introduce those changes now, I will be happy to send a
> series that changes that code as well.
It would be better if it and simplifying kvm_check_features_against_host()
were in here together.

> 
> > 
> > > 
> > > Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
> > > ---
> > >  target-i386/cpu.c | 2 ++
> > >  1 file changed, 2 insertions(+)
> > > 
> > > diff --git a/target-i386/cpu.c b/target-i386/cpu.c
> > > index 6e2d32d..76f19f0 100644
> > > --- a/target-i386/cpu.c
> > > +++ b/target-i386/cpu.c
> > > @@ -900,6 +900,8 @@ static void kvm_cpu_fill_host(x86_def_t *x86_cpu_def)
> > >      /* Other KVM-specific feature fields: */
> > >      x86_cpu_def->svm_features =
> > >                  kvm_arch_get_supported_cpuid(s, 0x8000000A, 0, R_EDX);
> > > +    x86_cpu_def->kvm_features =
> > > +                kvm_arch_get_supported_cpuid(s, KVM_CPUID_FEATURES, 0,
> > > R_EAX); 
> > >  #endif /* CONFIG_KVM */
> > >  }
> > 
> 
> -- 
> Eduardo
>
Eduardo Habkost - Jan. 2, 2013, 8:52 p.m.
On Wed, Jan 02, 2013 at 09:30:20PM +0100, Igor Mammedov wrote:
> On Wed, 2 Jan 2013 13:29:10 -0200
> Eduardo Habkost <ehabkost@redhat.com> wrote:
> 
> > On Wed, Jan 02, 2013 at 03:52:45PM +0100, Igor Mammedov wrote:
> > > On Fri, 28 Dec 2012 16:37:34 -0200
> > > Eduardo Habkost <ehabkost@redhat.com> wrote:
> > > 
> > > > When using -cpu host, we don't need to use the kvm_default_features
> > > > variable, as the user is explicitly asking QEMU to enable all feature
> > > > supported by the host.
> > > > 
> > > > This changes the kvm_cpu_fill_host() code to use GET_SUPPORTED_CPUID to
> > > > initialize the kvm_features field, so we get all host KVM features
> > > > enabled.
> > > 
> > > 1_2 and 1_3 compat machines diff on pv_eoi flag, with this patch 1_2 might
> > > have it set.
> > > Is it ok from compat machines pov?
> > 
> > -cpu host is completely dependent on host hardware and kernel version,
> > there are no compatibility expectations.
> 
> It's still kind of unpleasant surprise if on the same host
> "qemu-1.3 -cpu host -machine pc-1.2" and "qemu-1.3+ -cpu host -machine pc-1.2"
> would give different pv_eoi feature default, where pv-eoi should be
> available after -machine pc-1.2 by default.

Just like you may end up getting new features enabled by -cpu host after
upgrading the kernel, you may end up getting new features enabled by
-cpu host after upgrading qemu. If you don't like surprises, don't use
-cpu host.  ;-)

I don't think machine-types exist to avoid user surprise, they exist to
keep compatibility (which is not expected to happen when using -cpu
host). Keeping compatibility is hard enough in the cases where we really
need it, I don't think it is worth the extra work and complexity for an
use case where compatibility is not expected.

> 
> > 
> > > 
> > > > 
> > > > This will also allow use to properly check/enforce KVM features inside
> > > > kvm_check_features_against_host() later. For example, we will be able to
> > > > make this:
> > > > 
> > > >   $ qemu-system-x86_64 -cpu ...,+kvm_pv_eoi,enforce
> > > > 
> > > > refuse to start if kvm_pv_eoi is not supported by the host (after we fix
> > > > kvm_check_features_against_host() to check KVM flags as well).
> > > It would be nice to have kvm_check_features_against_host() patch in this
> > > series to verify that this patch and previous patch works as expected.
> > 
> > The kvm_check_features_against_host() change would be a user-visible
> > behavior change, and I wanted to keep the changes minimal by now. (the
> > main reason I submitted this earlier is to make it easier to clean up
> > the init code for CPU subclasses)
> > 
> > I was planning to introduce those behavior changes only after
> > introducing the feature-word array, so the kvm_check_features_against_host()
> > code can become simpler and easier to review (instead of adding 4
> > additional items to the messy struct model_features_t array). But if you
> > think we can introduce those changes now, I will be happy to send a
> > series that changes that code as well.
> It would be better if it and simplifying kvm_check_features_against_host()
> were in here together.

The best way I see to simplify kvm_check_features_against_host()
requires the feature words array patch, that touches _lots_ of code. I
wanted to avoid adding such an intrusive patch as a dependency.

> 
> > 
> > > 
> > > > 
> > > > Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
> > > > ---
> > > >  target-i386/cpu.c | 2 ++
> > > >  1 file changed, 2 insertions(+)
> > > > 
> > > > diff --git a/target-i386/cpu.c b/target-i386/cpu.c
> > > > index 6e2d32d..76f19f0 100644
> > > > --- a/target-i386/cpu.c
> > > > +++ b/target-i386/cpu.c
> > > > @@ -900,6 +900,8 @@ static void kvm_cpu_fill_host(x86_def_t *x86_cpu_def)
> > > >      /* Other KVM-specific feature fields: */
> > > >      x86_cpu_def->svm_features =
> > > >                  kvm_arch_get_supported_cpuid(s, 0x8000000A, 0, R_EDX);
> > > > +    x86_cpu_def->kvm_features =
> > > > +                kvm_arch_get_supported_cpuid(s, KVM_CPUID_FEATURES, 0,
> > > > R_EAX); 
> > > >  #endif /* CONFIG_KVM */
> > > >  }
> > > 
> > 
> > -- 
> > Eduardo
> > 
> 
> 
> -- 
> Regards,
>   Igor

Patch

diff --git a/target-i386/cpu.c b/target-i386/cpu.c
index 6e2d32d..76f19f0 100644
--- a/target-i386/cpu.c
+++ b/target-i386/cpu.c
@@ -900,6 +900,8 @@  static void kvm_cpu_fill_host(x86_def_t *x86_cpu_def)
     /* Other KVM-specific feature fields: */
     x86_cpu_def->svm_features =
                 kvm_arch_get_supported_cpuid(s, 0x8000000A, 0, R_EDX);
+    x86_cpu_def->kvm_features =
+                kvm_arch_get_supported_cpuid(s, KVM_CPUID_FEATURES, 0, R_EAX);
 
 #endif /* CONFIG_KVM */
 }