Re: [PATCH v2 04/37] powerpc/64s: remove KVM SKIP test from instruction breakpoint handler

Excerpts from Daniel Axtens's message of February 26, 2021 3:44 pm:

Nicholas Piggin [off-list ref] writes:

quoted

The code being executed in KVM_GUEST_MODE_SKIP is hypervisor code with
MSR[IR]=0, so the faults of concern are the d-side ones caused by access
to guest context by the hypervisor.

Instruction breakpoint interrupts are not a concern here. It's unlikely
any good would come of causing breaks in this code, but skipping the
instruction that caused it won't help matters (e.g., skip the mtmsr that
sets MSR[DR]=0 or clears KVM_GUEST_MODE_SKIP).

I'm not entirely clear on the example here, but the patch makes sense
and I can follow your logic for removing the IKVM_SKIP handler from the
instruction breakpoint exception.

The example just means that a breakpoint interrupt on any instruction 
inside the guest mode skip region would be skipped, and skipping one of 
those (mtmsrd or store that gets us out of guest mode skip) would cause 
a crash.

Thanks,
Nick

On that basis:
Reviewed-by: Daniel Axtens <redacted>

Kind regards,
Daniel

quoted

Reviewed-by: Fabiano Rosas <redacted>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
 arch/powerpc/kernel/exceptions-64s.S | 1 -
 1 file changed, 1 deletion(-)

diff --git a/arch/powerpc/kernel/exceptions-64s.S b/arch/powerpc/kernel/exceptions-64s.S
index a027600beeb1..0097e0676ed7 100644
--- a/arch/powerpc/kernel/exceptions-64s.S
+++ b/arch/powerpc/kernel/exceptions-64s.S

@@ -2553,7 +2553,6 @@ EXC_VIRT_NONE(0x5200, 0x100)
 INT_DEFINE_BEGIN(instruction_breakpoint)
 	IVEC=0x1300
 #ifdef CONFIG_KVM_BOOK3S_PR_POSSIBLE
-	IKVM_SKIP=1
 	IKVM_REAL=1
 #endif
 INT_DEFINE_END(instruction_breakpoint)

-- 
2.23.0