Re: [PATCH v2 03/21] KVM: x86: Don't check for code breakpoints when emulating on exception

[Jim Mattson]

On Tue, Jun 14, 2022 at 1:47 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> Don't check for code breakpoints during instruction emulation if the
> emulation was triggered by exception interception.  Code breakpoints are
> the highest priority fault-like exception, and KVM only emulates on
> exceptions that are fault-like.  Thus, if hardware signaled a different
> exception, then the vCPU is already passed the stage of checking for
> hardware breakpoints.
>
> This is likely a glorified nop in terms of functionality, and is more for
> clarification and is technically an optimization.  Intel's SDM explicitly
> states vmcs.GUEST_RFLAGS.RF on exception interception is the same as the
> value that would have been saved on the stack had the exception not been
> intercepted, i.e. will be '1' due to all fault-like exceptions setting RF
> to '1'.  AMD says "guest state saved ... is the processor state as of the
> moment the intercept triggers", but that begs the question, "when does
> the intercept trigger?".

IIRC, AMD does not prematurely clobber EFLAGS.RF on an intercepted exception.

This is actually a big deal with shadow paging. On Intel, the
hypervisor can't fully squash a #PF and restart the guest instruction
after filling in the shadow page table entry...not easily, anyway.

(OTOH, AMD does prematurely clobber DR6 and DR7 on an intercepted #DB.
So, no one should be celebrating!)