| Message ID | 20120711003454.GA22757@tyr.buserror.net (mailing list archive) |
|---|---|
| State | Not Applicable |
| Delegated to: | Scott Wood |
| Headers | show |
On Tue, 2012-07-10 at 19:34 -0500, Scott Wood wrote: > Unlike classic, we don't really need the MSR change to be atomic with the > branch. This eliminates a trap as a KVM guest (in the absence of > hardware hypervisor extensions), where mtmsr is paravirtualized but rfi > is not. For a virtualized guest without any paravirtualization, this > eliminates an additional two traps (SRR0/1). In fact, I wonder, what do we write into the MSR at this point that wasn't already in it in BookE ? RI ? I wonder if we could get away without the mtmsr alltogether... Cheers, Ben. > Signed-off-by: Scott Wood <scottwood@freescale.com> > --- > arch/powerpc/kernel/entry_32.S | 16 ++++++++++++++++ > 1 files changed, 16 insertions(+), 0 deletions(-) > > diff --git a/arch/powerpc/kernel/entry_32.S b/arch/powerpc/kernel/entry_32.S > index ba3aeb4..6bb637c 100644 > --- a/arch/powerpc/kernel/entry_32.S > +++ b/arch/powerpc/kernel/entry_32.S > @@ -193,6 +193,9 @@ transfer_to_handler_cont: > lwz r11,0(r9) /* virtual address of handler */ > lwz r9,4(r9) /* where to go when done */ > #ifdef CONFIG_TRACE_IRQFLAGS > +#ifdef CONFIG_BOOKE > + mtmsr r10 > +#else > lis r12,reenable_mmu@h > ori r12,r12,reenable_mmu@l > mtspr SPRN_SRR0,r12 > @@ -201,6 +204,7 @@ transfer_to_handler_cont: > RFI > reenable_mmu: /* re-enable mmu so we can */ > mfmsr r10 > +#endif /* !CONFIG_BOOKE */ > lwz r12,_MSR(r1) > xor r10,r10,r12 > andi. r10,r10,MSR_EE /* Did EE change? */ > @@ -247,11 +251,23 @@ reenable_mmu: /* re-enable mmu so we can */ > mtlr r9 > bctr /* jump to handler */ > #else /* CONFIG_TRACE_IRQFLAGS */ > +#ifdef CONFIG_BOOKE > + /* > + * We're not changing address space on Book E, and the extra rfi > + * can hurt when virtualized without hardware support -- whereas > + * mtmsr can be paravirtualized. > + */ > + mtmsr r10 > + mtctr r11 > + mtlr r9 > + bctr > +#else > mtspr SPRN_SRR0,r11 > mtspr SPRN_SRR1,r10 > mtlr r9 > SYNC > RFI /* jump to handler, enable MMU */ > +#endif /* !CONFIG_BOOKE */ > #endif /* CONFIG_TRACE_IRQFLAGS */ > > #if defined (CONFIG_6xx) || defined(CONFIG_E500)
On 07/10/2012 07:36 PM, Benjamin Herrenschmidt wrote: > On Tue, 2012-07-10 at 19:34 -0500, Scott Wood wrote: >> Unlike classic, we don't really need the MSR change to be atomic with the >> branch. This eliminates a trap as a KVM guest (in the absence of >> hardware hypervisor extensions), where mtmsr is paravirtualized but rfi >> is not. For a virtualized guest without any paravirtualization, this >> eliminates an additional two traps (SRR0/1). > > In fact, I wonder, what do we write into the MSR at this point that > wasn't already in it in BookE ? RI ? I wonder if we could get away > without the mtmsr alltogether... Doesn't EE get set there for some exceptions? -Scott
On 11.07.2012, at 02:34, Scott Wood wrote: > Unlike classic, we don't really need the MSR change to be atomic with the > branch. This eliminates a trap as a KVM guest (in the absence of > hardware hypervisor extensions), where mtmsr is paravirtualized but rfi > is not. For a virtualized guest without any paravirtualization, this > eliminates an additional two traps (SRR0/1). > > Signed-off-by: Scott Wood <scottwood@freescale.com> > --- > arch/powerpc/kernel/entry_32.S | 16 ++++++++++++++++ > 1 files changed, 16 insertions(+), 0 deletions(-) > > diff --git a/arch/powerpc/kernel/entry_32.S b/arch/powerpc/kernel/entry_32.S > index ba3aeb4..6bb637c 100644 > --- a/arch/powerpc/kernel/entry_32.S > +++ b/arch/powerpc/kernel/entry_32.S > @@ -193,6 +193,9 @@ transfer_to_handler_cont: > lwz r11,0(r9) /* virtual address of handler */ > lwz r9,4(r9) /* where to go when done */ > #ifdef CONFIG_TRACE_IRQFLAGS > +#ifdef CONFIG_BOOKE > + mtmsr r10 > +#else > lis r12,reenable_mmu@h > ori r12,r12,reenable_mmu@l > mtspr SPRN_SRR0,r12 > @@ -201,6 +204,7 @@ transfer_to_handler_cont: > RFI > reenable_mmu: /* re-enable mmu so we can */ > mfmsr r10 > +#endif /* !CONFIG_BOOKE */ > lwz r12,_MSR(r1) > xor r10,r10,r12 > andi. r10,r10,MSR_EE /* Did EE change? */ > @@ -247,11 +251,23 @@ reenable_mmu: /* re-enable mmu so we can */ > mtlr r9 > bctr /* jump to handler */ > #else /* CONFIG_TRACE_IRQFLAGS */ > +#ifdef CONFIG_BOOKE > + /* > + * We're not changing address space on Book E, and the extra rfi > + * can hurt when virtualized without hardware support -- whereas > + * mtmsr can be paravirtualized. We can always paravirtualize RFI as well if it makes sense. Alex
On 07/10/2012 07:44 PM, Alexander Graf wrote: > > On 11.07.2012, at 02:34, Scott Wood wrote: >> +#ifdef CONFIG_BOOKE >> + /* >> + * We're not changing address space on Book E, and the extra rfi >> + * can hurt when virtualized without hardware support -- whereas >> + * mtmsr can be paravirtualized. > > We can always paravirtualize RFI as well if it makes sense. I'm not sure that's possible. We thought about it a while back, but IIRC the difficulty was not leaving a register clobbered. -Scott
On Tue, 2012-07-10 at 19:41 -0500, Scott Wood wrote: > On 07/10/2012 07:36 PM, Benjamin Herrenschmidt wrote: > > On Tue, 2012-07-10 at 19:34 -0500, Scott Wood wrote: > >> Unlike classic, we don't really need the MSR change to be atomic with the > >> branch. This eliminates a trap as a KVM guest (in the absence of > >> hardware hypervisor extensions), where mtmsr is paravirtualized but rfi > >> is not. For a virtualized guest without any paravirtualization, this > >> eliminates an additional two traps (SRR0/1). > > > > In fact, I wonder, what do we write into the MSR at this point that > > wasn't already in it in BookE ? RI ? I wonder if we could get away > > without the mtmsr alltogether... > > Doesn't EE get set there for some exceptions? It does, tho arguably it shouldn't in most cases :-) I'm happy to turn a bunch of these into explicit local_irq_enable() in the C code though which will turn into a wrteei which is more efficient on BookE. Cheers, Ben.
On Tue, 2012-07-10 at 19:47 -0500, Scott Wood wrote: > On 07/10/2012 07:44 PM, Alexander Graf wrote: > > > > On 11.07.2012, at 02:34, Scott Wood wrote: > >> +#ifdef CONFIG_BOOKE > >> + /* > >> + * We're not changing address space on Book E, and the extra rfi > >> + * can hurt when virtualized without hardware support -- whereas > >> + * mtmsr can be paravirtualized. > > > > We can always paravirtualize RFI as well if it makes sense. > > I'm not sure that's possible. We thought about it a while back, but > IIRC the difficulty was not leaving a register clobbered. Besides mtmsr is slow on real HW as well. Also paravirt as done today for complex instructions like mtmsr is racy :-) (I already had a chat about that with Alex a while back, we might want to re-consider what kind of fix can be done at some point). Cheers, Ben.
diff --git a/arch/powerpc/kernel/entry_32.S b/arch/powerpc/kernel/entry_32.S index ba3aeb4..6bb637c 100644 --- a/arch/powerpc/kernel/entry_32.S +++ b/arch/powerpc/kernel/entry_32.S @@ -193,6 +193,9 @@ transfer_to_handler_cont: lwz r11,0(r9) /* virtual address of handler */ lwz r9,4(r9) /* where to go when done */ #ifdef CONFIG_TRACE_IRQFLAGS +#ifdef CONFIG_BOOKE + mtmsr r10 +#else lis r12,reenable_mmu@h ori r12,r12,reenable_mmu@l mtspr SPRN_SRR0,r12 @@ -201,6 +204,7 @@ transfer_to_handler_cont: RFI reenable_mmu: /* re-enable mmu so we can */ mfmsr r10 +#endif /* !CONFIG_BOOKE */ lwz r12,_MSR(r1) xor r10,r10,r12 andi. r10,r10,MSR_EE /* Did EE change? */ @@ -247,11 +251,23 @@ reenable_mmu: /* re-enable mmu so we can */ mtlr r9 bctr /* jump to handler */ #else /* CONFIG_TRACE_IRQFLAGS */ +#ifdef CONFIG_BOOKE + /* + * We're not changing address space on Book E, and the extra rfi + * can hurt when virtualized without hardware support -- whereas + * mtmsr can be paravirtualized. + */ + mtmsr r10 + mtctr r11 + mtlr r9 + bctr +#else mtspr SPRN_SRR0,r11 mtspr SPRN_SRR1,r10 mtlr r9 SYNC RFI /* jump to handler, enable MMU */ +#endif /* !CONFIG_BOOKE */ #endif /* CONFIG_TRACE_IRQFLAGS */ #if defined (CONFIG_6xx) || defined(CONFIG_E500)
Unlike classic, we don't really need the MSR change to be atomic with the branch. This eliminates a trap as a KVM guest (in the absence of hardware hypervisor extensions), where mtmsr is paravirtualized but rfi is not. For a virtualized guest without any paravirtualization, this eliminates an additional two traps (SRR0/1). Signed-off-by: Scott Wood <scottwood@freescale.com> --- arch/powerpc/kernel/entry_32.S | 16 ++++++++++++++++ 1 files changed, 16 insertions(+), 0 deletions(-)