Re: [PATCH] powerpc/booke: Eliminate rfi from exception entry path.
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Date: 2012-07-11 00:36:56
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.
quoted hunk ↗ jump to hunk
Signed-off-by: Scott Wood <redacted> --- 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)