Re: [PATCH 3/6] KVM: x86: Add dedicated emulator helper for grabbing CPUID.maxphyaddr

[Jan Kiszka <jan.kiszka@xxxxxxxxxxx>]

On 03.03.20 09:48, Paolo Bonzini wrote:
> On 02/03/20 20:57, Sean Christopherson wrote:
> > Add a helper to retrieve cpuid_maxphyaddr() instead of manually
> > calculating the value in the emulator via raw CPUID output.  In addition
> > to consolidating logic, this also paves the way toward simplifying
> > kvm_cpuid(), whose somewhat confusing return value exists purely to
> > support the emulator's maxphyaddr calculation.
> >
> > No functional change intended.
>
> I don't think this is a particularly useful change.  Yes, it's not
> intuitive but is it more than a matter of documentation (and possibly
> moving the check_cr_write snippet into a separate function)?

Besides the non obvious return value of the current function, this 
approach also avoids leaving cpuid traces for querying maxphyaddr, which 
is also not very intuitive IMHO.

Jan

> Paolo
>
> > Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> > ---
> >   arch/x86/include/asm/kvm_emulate.h |  1 +
> >   arch/x86/kvm/emulate.c             | 10 +---------
> >   arch/x86/kvm/x86.c                 |  6 ++++++
> >   3 files changed, 8 insertions(+), 9 deletions(-)
> >
> > diff --git a/arch/x86/include/asm/kvm_emulate.h b/arch/x86/include/asm/kvm_emulate.h
> > index bf5f5e476f65..ded06515d30f 100644
> > --- a/arch/x86/include/asm/kvm_emulate.h
> > +++ b/arch/x86/include/asm/kvm_emulate.h
> > @@ -222,6 +222,7 @@ struct x86_emulate_ops {
> >   
> >   	bool (*get_cpuid)(struct x86_emulate_ctxt *ctxt, u32 *eax, u32 *ebx,
> >   			  u32 *ecx, u32 *edx, bool check_limit);
> > +	int (*get_cpuid_maxphyaddr)(struct x86_emulate_ctxt *ctxt);
> >   	bool (*guest_has_long_mode)(struct x86_emulate_ctxt *ctxt);
> >   	bool (*guest_has_movbe)(struct x86_emulate_ctxt *ctxt);
> >   	bool (*guest_has_fxsr)(struct x86_emulate_ctxt *ctxt);
> > diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> > index dd19fb3539e0..bf02ed51e90f 100644
> > --- a/arch/x86/kvm/emulate.c
> > +++ b/arch/x86/kvm/emulate.c
> > @@ -4244,16 +4244,8 @@ static int check_cr_write(struct x86_emulate_ctxt *ctxt)
> >   
> >   		ctxt->ops->get_msr(ctxt, MSR_EFER, &efer);
> >   		if (efer & EFER_LMA) {
> > -			u64 maxphyaddr;
> > -			u32 eax, ebx, ecx, edx;
> > +			int maxphyaddr = ctxt->ops->get_cpuid_maxphyaddr(ctxt);
> >   
> > -			eax = 0x80000008;
> > -			ecx = 0;
> > -			if (ctxt->ops->get_cpuid(ctxt, &eax, &ebx, &ecx,
> > -						 &edx, false))
> > -				maxphyaddr = eax & 0xff;
> > -			else
> > -				maxphyaddr = 36;
> >   			rsvd = rsvd_bits(maxphyaddr, 63);
> >   			if (ctxt->ops->get_cr(ctxt, 4) & X86_CR4_PCIDE)
> >   				rsvd &= ~X86_CR3_PCID_NOFLUSH;
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index ddd1d296bd20..5467ee71c25b 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -6209,6 +6209,11 @@ static bool emulator_get_cpuid(struct x86_emulate_ctxt *ctxt,
> >   	return kvm_cpuid(emul_to_vcpu(ctxt), eax, ebx, ecx, edx, check_limit);
> >   }
> >   
> > +static int emulator_get_cpuid_maxphyaddr(struct x86_emulate_ctxt *ctxt)
> > +{
> > +	return cpuid_maxphyaddr(emul_to_vcpu(ctxt));
> > +}
> > +
> >   static bool emulator_guest_has_long_mode(struct x86_emulate_ctxt *ctxt)
> >   {
> >   	return guest_cpuid_has(emul_to_vcpu(ctxt), X86_FEATURE_LM);
> > @@ -6301,6 +6306,7 @@ static const struct x86_emulate_ops emulate_ops = {
> >   	.fix_hypercall       = emulator_fix_hypercall,
> >   	.intercept           = emulator_intercept,
> >   	.get_cpuid           = emulator_get_cpuid,
> > +	.get_cpuid_maxphyaddr= emulator_get_cpuid_maxphyaddr,
> >   	.guest_has_long_mode = emulator_guest_has_long_mode,
> >   	.guest_has_movbe     = emulator_guest_has_movbe,
> >   	.guest_has_fxsr      = emulator_guest_has_fxsr,

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