Re: [PATCH 4/4] KVM: x86: Use 16-bit fields to track dirty/valid emulator GPRs

[Kees Cook]

On Wed, May 25, 2022 at 10:26:04PM +0000, Sean Christopherson wrote:
> Use a u16 instead of a u32 to track the dirty/valid status of GPRs in the
> emulator.  Unlike struct kvm_vcpu_arch, x86_emulate_ctxt tracks only the
> "true" GPRs, i.e. doesn't include RIP in its array, and so only needs to
> track 16 registers.
> 
> Note, having 16 GPRs is a fundamental property of x86-64 and will not
> change barring a massive architecture update.  Legacy x86 ModRM and SIB
> encodings use 3 bits for GPRs, i.e. support 8 registers.  x86-64 uses a
> single bit in the REX prefix for each possible reference type to double
> the number of supported GPRs to 16 registers (4 bits).
> 
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

-- 
Kees Cook