Re: [PATCH] powerpc/booke: Eliminate rfi from exception entry path.
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. ___ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev
Re: [PATCH] powerpc/booke: Eliminate rfi from exception entry path.
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. ___ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev
Re: [PATCH] powerpc/booke: Eliminate rfi from exception entry path.
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 ___ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev
Re: [PATCH] powerpc/booke: Eliminate rfi from exception entry path.
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 > --- > 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 */ > mtlrr9 > 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 ___ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev
Re: [PATCH] powerpc/booke: Eliminate rfi from exception entry path.
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 ___ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev
Re: [PATCH] powerpc/booke: Eliminate rfi from exception entry path.
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 > --- > 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 */ > mtlrr9 > 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 > + mtlrr9 > + bctr > +#else > mtspr SPRN_SRR0,r11 > mtspr SPRN_SRR1,r10 > mtlrr9 > SYNC > RFI /* jump to handler, enable MMU */ > +#endif /* !CONFIG_BOOKE */ > #endif /* CONFIG_TRACE_IRQFLAGS */ > > #if defined (CONFIG_6xx) || defined(CONFIG_E500) ___ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev
