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[RFC,v0] kvm: powerpc: Allow reuse of vCPU object

Message ID 1425879055-20628-1-git-send-email-bharata.rao@gmail.com (mailing list archive)
State Not Applicable
Delegated to: Michael Ellerman
Headers show

Commit Message

Bharata B Rao March 9, 2015, 5:30 a.m. UTC 
From: Bharata B Rao <bharata@linux.vnet.ibm.com>

Since KVM isn't equipped to handle closure of vcpu fd from userspace(QEMU)
correctly, certain work arounds have to be employed to allow reuse of
vcpu array slot in KVM during cpu hot plug/unplug from guest. One such
proposed workaround is to park the vcpu fd in userspace during cpu unplug
and reuse it later during next hotplug.

More details can be found here:
KVM: https://www.mail-archive.com/kvm@vger.kernel.org/msg102839.html
QEMU: http://lists.gnu.org/archive/html/qemu-devel/2014-12/msg00859.html

In order to support this workaround with PowerPC KVM, don't create or
initialize ICP if the vCPU is found to be already associated with an ICP.

Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
---
Note: It is not sure at the moment if "park vcpu and reuse" approach will
be acceptable to KVM/QEMU community, but nevertheless I wanted to check
if this little patch is harmful or not.

 arch/powerpc/kvm/book3s_xics.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

Comments

Bharata B Rao March 16, 2015, 3:25 a.m. UTC | #1 
Any feedback on the below patch ?

On Mon, Mar 9, 2015 at 11:00 AM,  <bharata.rao@gmail.com> wrote:
> From: Bharata B Rao <bharata@linux.vnet.ibm.com>
>
> Since KVM isn't equipped to handle closure of vcpu fd from userspace(QEMU)
> correctly, certain work arounds have to be employed to allow reuse of
> vcpu array slot in KVM during cpu hot plug/unplug from guest. One such
> proposed workaround is to park the vcpu fd in userspace during cpu unplug
> and reuse it later during next hotplug.
>
> More details can be found here:
> KVM: https://www.mail-archive.com/kvm@vger.kernel.org/msg102839.html
> QEMU: http://lists.gnu.org/archive/html/qemu-devel/2014-12/msg00859.html
>
> In order to support this workaround with PowerPC KVM, don't create or
> initialize ICP if the vCPU is found to be already associated with an ICP.
>
> Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
> ---
> Note: It is not sure at the moment if "park vcpu and reuse" approach will
> be acceptable to KVM/QEMU community, but nevertheless I wanted to check
> if this little patch is harmful or not.
>
>  arch/powerpc/kvm/book3s_xics.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/arch/powerpc/kvm/book3s_xics.c b/arch/powerpc/kvm/book3s_xics.c
> index a4a8d9f..ead3a35 100644
> --- a/arch/powerpc/kvm/book3s_xics.c
> +++ b/arch/powerpc/kvm/book3s_xics.c
> @@ -1313,8 +1313,13 @@ int kvmppc_xics_connect_vcpu(struct kvm_device *dev, struct kvm_vcpu *vcpu,
>                 return -EPERM;
>         if (xics->kvm != vcpu->kvm)
>                 return -EPERM;
> -       if (vcpu->arch.irq_type)
> -               return -EBUSY;
> +
> +       /*
> +        * If irq_type is already set, don't reinialize but
> +        * return success allowing this vcpu to be reused.
> +        */
> +       if (vcpu->arch.irq_type != KVMPPC_IRQ_DEFAULT)
> +               return 0;
>
>         r = kvmppc_xics_create_icp(vcpu, xcpu);
>         if (!r)
> --
> 2.1.0
>
diff mbox

Patch

diff --git a/arch/powerpc/kvm/book3s_xics.c b/arch/powerpc/kvm/book3s_xics.c
index a4a8d9f..ead3a35 100644
--- a/arch/powerpc/kvm/book3s_xics.c
+++ b/arch/powerpc/kvm/book3s_xics.c
@@ -1313,8 +1313,13 @@  int kvmppc_xics_connect_vcpu(struct kvm_device *dev, struct kvm_vcpu *vcpu,
 		return -EPERM;
 	if (xics->kvm != vcpu->kvm)
 		return -EPERM;
-	if (vcpu->arch.irq_type)
-		return -EBUSY;
+
+	/*
+	 * If irq_type is already set, don't reinialize but
+	 * return success allowing this vcpu to be reused.
+	 */
+	if (vcpu->arch.irq_type != KVMPPC_IRQ_DEFAULT)
+		return 0;
 
 	r = kvmppc_xics_create_icp(vcpu, xcpu);
 	if (!r)