Re: [PATCH] KVM: selftests: x86: Prioritize getting max_gfn from GuestPhysBits

[Tao Su <tao1.su@linux.intel.com>]

On Wed, May 08, 2024 at 07:09:48AM -0700, Sean Christopherson wrote:
> On Wed, May 08, 2024, Tao Su wrote:
> > Use the max mappable GPA via GuestPhysBits advertised by KVM to calculate
> > max_gfn. Currently some selftests (e.g. access_tracking_perf_test,
> > dirty_log_test...) add RAM regions close to max_gfn, so guest may access
> > GPA beyond its mappable range and cause infinite loop.
> > 
> > Prioritize obtaining pa_bits from the GuestPhysBits field advertised by
> > KVM, so max_gfn can be limited to the mappable range.
> > 
> > Signed-off-by: Tao Su <tao1.su@linux.intel.com>
> > ---
> > This patch is based on https://github.com/kvm-x86/linux/commit/b628cb523c65
> > ---
> >  tools/testing/selftests/kvm/include/x86_64/processor.h | 1 +
> >  tools/testing/selftests/kvm/lib/x86_64/processor.c     | 4 +++-
> >  2 files changed, 4 insertions(+), 1 deletion(-)
> > 
> > diff --git a/tools/testing/selftests/kvm/include/x86_64/processor.h b/tools/testing/selftests/kvm/include/x86_64/processor.h
> > index 81ce37ec407d..ff99f66d81a0 100644
> > --- a/tools/testing/selftests/kvm/include/x86_64/processor.h
> > +++ b/tools/testing/selftests/kvm/include/x86_64/processor.h
> > @@ -282,6 +282,7 @@ struct kvm_x86_cpu_property {
> >  #define X86_PROPERTY_MAX_EXT_LEAF		KVM_X86_CPU_PROPERTY(0x80000000, 0, EAX, 0, 31)
> >  #define X86_PROPERTY_MAX_PHY_ADDR		KVM_X86_CPU_PROPERTY(0x80000008, 0, EAX, 0, 7)
> >  #define X86_PROPERTY_MAX_VIRT_ADDR		KVM_X86_CPU_PROPERTY(0x80000008, 0, EAX, 8, 15)
> > +#define X86_PROPERTY_MAX_GUEST_PHY_ADDR		KVM_X86_CPU_PROPERTY(0x80000008, 0, EAX, 16, 23)
> >  #define X86_PROPERTY_SEV_C_BIT			KVM_X86_CPU_PROPERTY(0x8000001F, 0, EBX, 0, 5)
> >  #define X86_PROPERTY_PHYS_ADDR_REDUCTION	KVM_X86_CPU_PROPERTY(0x8000001F, 0, EBX, 6, 11)
> >  
> > diff --git a/tools/testing/selftests/kvm/lib/x86_64/processor.c b/tools/testing/selftests/kvm/lib/x86_64/processor.c
> > index 74a4c736c9ae..6c69f1dfeed2 100644
> > --- a/tools/testing/selftests/kvm/lib/x86_64/processor.c
> > +++ b/tools/testing/selftests/kvm/lib/x86_64/processor.c
> > @@ -1074,7 +1074,9 @@ void kvm_get_cpu_address_width(unsigned int *pa_bits, unsigned int *va_bits)
> >  		*pa_bits = kvm_cpu_has(X86_FEATURE_PAE) ? 36 : 32;
> >  		*va_bits = 32;
> >  	} else {
> > -		*pa_bits = kvm_cpu_property(X86_PROPERTY_MAX_PHY_ADDR);
> > +		*pa_bits = kvm_cpu_property(X86_PROPERTY_MAX_GUEST_PHY_ADDR);
> > +		if (*pa_bits == 0)
> > +			*pa_bits = kvm_cpu_property(X86_PROPERTY_MAX_PHY_ADDR);
> 
> This is the wrong place to incorporaate the max mappable GPA.  The pa_bits field
> should reflect the "real" MAXPHYADDR, it's vm->max_gfn that needs to be adjusted,
> and x86 selftests already overrides vm_compute_max_gfn() specifically to deal with
> goofy edge cases like this.

Thanks for pointing out! I will adjust max_gfn in the vm_compute_max_gfn() overrided
by x86 selftests.

> 
> >  		*va_bits = kvm_cpu_property(X86_PROPERTY_MAX_VIRT_ADDR);
> >  	}
> >  }
> > 
> > base-commit: dccb07f2914cdab2ac3a5b6c98406f765acab803
> > -- 
> > 2.34.1
> >