Re: [PATCH v2] KVM: x86/pt: Ignore all unknown Intel PT capabilities

[Xiaoyao Li]

On 1/11/2022 12:20 PM, Like Xu wrote:
> On 11/1/2022 8:57 am, Sean Christopherson wrote:
> > On Mon, Jan 10, 2022, Like Xu wrote:
> > > From: Like Xu <likexu@xxxxxxxxxxx>
> > >
> > > Some of the new Intel PT capabilities (e.g. SDM Vol3, 32.2.4 Event
> > > Tracing, it exposes details about the asynchronous events, when they are
> > > generated, and when their corresponding software event handler completes
> > > execution) cannot be safely and fully emulated by the KVM, especially
> > > emulating the simultaneous writing of guest PT packets generated by
> > > the KVM to the guest PT buffer.
> > >
> > > For KVM, it's better to advertise currently supported features based on
> > > the "static struct pt_cap_desc" implemented in the host PT driver and
> > > ignore _all_ unknown features before they have been investigated one by
> > > one and supported in a safe manner, leaving the rest as system-wide-only
> > > tracing capabilities.
> > >
> > > Suggested-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> > > Signed-off-by: Like Xu <likexu@xxxxxxxxxxx>
> > > ---
> > > v1 -> v2 Changelog:
> > > - Be safe and ignore _all_ unknown capabilities. (Paolo)
> > >
> > > Previous:
> > > https://lore.kernel.org/kvm/20220106085533.84356-1-likexu@xxxxxxxxxxx/
> > >
> > >   arch/x86/kvm/cpuid.c | 2 ++
> > >   1 file changed, 2 insertions(+)
> > >
> > > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> > > index 0b920e12bb6d..439b93359848 100644
> > > --- a/arch/x86/kvm/cpuid.c
> > > +++ b/arch/x86/kvm/cpuid.c
> > > @@ -901,6 +901,8 @@ static inline int __do_cpuid_func(struct 
> > > kvm_cpuid_array *array, u32 function)
> > >               break;
> > >           }
> > > +        /* It's better to be safe and ignore _all_ unknown 
> > > capabilities. */
> >
> > No need to justify why unknown capabilities are hidden as that's very 
> > much (supposed
> > to be) standard KVM behavior.
> >
> > > +        entry->ebx &= GENMASK(5, 0);
> >
> > Please add a #define somewhere so that this is self-documenting, e.g. see
> > KVM_SUPPORTED_XCR0.
>
> How about we define this macro in the <asm/intel_pt.h> so that the next 
> PT capability
> enabler can update the mask with minimal effort, considering that many 
> pure kernel
> developers don't care about KVM code ?
>
> > And why just EBX?  ECX appears to enumerate features too, and EDX is 
> > presumably
> > reserved to enumerate yet more features when EBX/ECX run out of bits.
>
> Yes, how about this version:
>
> diff --git a/arch/x86/include/asm/intel_pt.h 
> b/arch/x86/include/asm/intel_pt.h
> index ebe8d2ea44fe..da94d0eeb9df 100644
> --- a/arch/x86/include/asm/intel_pt.h
> +++ b/arch/x86/include/asm/intel_pt.h
> @@ -24,6 +24,12 @@ enum pt_capabilities {
>       PT_CAP_psb_periods,
>   };
>
> +#define GUEST_SUPPORTED_CPUID_14_EBX    \
> +    (BIT(0) | BIT(1) | BIT(2) | BIT(3) | BIT(4) | BIT(5))
> +
> +#define GUEST_SUPPORTED_CPUID_14_ECX    \
> +    (BIT(0) | BIT(1) | BIT(2) | BIT(3) | BIT(31))
> +

I doubt BIT(3) of CPUID_14_ECX can be exposed to guest directly.

It means "output to Trace Transport Subsystem Supported". If I 
understand correctly, it at least needs passthrough of the said 
Transport Subsystem or emulation of it.

>   #if defined(CONFIG_PERF_EVENTS) && defined(CONFIG_CPU_SUP_INTEL)
>   void cpu_emergency_stop_pt(void);
>   extern u32 intel_pt_validate_hw_cap(enum pt_capabilities cap);
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 0b920e12bb6d..be8c9170f98e 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -19,6 +19,7 @@
>   #include <asm/user.h>
>   #include <asm/fpu/xstate.h>
>   #include <asm/sgx.h>
> +#include <asm/intel_pt.h>
>   #include "cpuid.h"
>   #include "lapic.h"
>   #include "mmu.h"
> @@ -900,7 +901,10 @@ static inline int __do_cpuid_func(struct 
> kvm_cpuid_array *array, u32 function)
>               entry->eax = entry->ebx = entry->ecx = entry->edx = 0;
>               break;
>           }
> -
> +        entry->eax = min(entry->eax, 1u);
> +        entry->ebx &= GUEST_SUPPORTED_CPUID_14_EBX;
> +        entry->ecx &= GUEST_SUPPORTED_CPUID_14_ECX;
> +        entry->edx = 0;
>           for (i = 1, max_idx = entry->eax; i <= max_idx; ++i) {
>               if (!do_host_cpuid(array, function, i))
>                   goto out;