Re: [PATCH v2 3/8] x86/msr: explicitly handle AMD DE_CFG

["Roger Pau Monné" <roger.pau@citrix.com>]

On Thu, Aug 20, 2020 at 06:08:53PM +0100, Andrew Cooper wrote:
> On 20/08/2020 16:08, Roger Pau Monne wrote:
> 
> > diff --git a/xen/arch/x86/msr.c b/xen/arch/x86/msr.c
> > index ca4307e19f..a890cb9976 100644
> > --- a/xen/arch/x86/msr.c
> > +++ b/xen/arch/x86/msr.c
> > @@ -274,6 +274,14 @@ int guest_rdmsr(struct vcpu *v, uint32_t msr, uint64_t *val)
> >          *val = msrs->tsc_aux;
> >          break;
> >  
> > +    case MSR_AMD64_DE_CFG:
> > +        if ( !(cp->x86_vendor & (X86_VENDOR_AMD | X86_VENDOR_HYGON)) ||
> > +             !(boot_cpu_data.x86_vendor & (X86_VENDOR_AMD |
> > +                                           X86_VENDOR_HYGON)) ||
> > +             rdmsr_safe(MSR_AMD64_DE_CFG, *val) )
> > +            goto gp_fault;
> > +        break;
> 
> Ah.  What I intended was to read just bit 2 and nothing else.
> 
> Leaking the full value is non-ideal from a migration point of view, and
> in this case, you can avoid querying hardware entirely.
> 
> Just return AMD64_DE_CFG_LFENCE_SERIALISE here.  The only case where it
> won't be true is when the hypervisor running us (i.e. Xen) failed to set
> it up, and the CPU boot path failed to adjust it, at which point the
> whole system has much bigger problems.

Right, the rest are just model specific workarounds AFAICT, so it's
safe to not display them. A guest might attempt to set them, but we
should simply drop the write, see below.

> 
> > +
> >      case MSR_AMD64_DR0_ADDRESS_MASK:
> >      case MSR_AMD64_DR1_ADDRESS_MASK ... MSR_AMD64_DR3_ADDRESS_MASK:
> >          if ( !cp->extd.dbext )
> > @@ -499,6 +507,12 @@ int guest_wrmsr(struct vcpu *v, uint32_t msr, uint64_t val)
> >              wrmsr_tsc_aux(val);
> >          break;
> >  
> > +    case MSR_AMD64_DE_CFG:
> > +        if ( !(cp->x86_vendor & (X86_VENDOR_AMD | X86_VENDOR_HYGON)) ||
> > +             !(boot_cpu_data.x86_vendor & (X86_VENDOR_AMD | X86_VENDOR_HYGON)) )
> > +            goto gp_fault;
> > +        break;
> 
> There should be no problem yielding #GP here (i.e. dropping this hunk).
> 
> IIRC, it was the behaviour of certain hypervisors when Spectre hit, so
> all guests ought to cope.  (And indeed, not try to redundantly set the
> bit to start with).

It seems like OpenBSD will try to do so unconditionally, see:

https://www.illumos.org/issues/12998

According to the report there returning #GP when trying to WRMSR
DE_CFG will cause OpenBSD to panic, so I think we need to keep this
behavior of silently dropping writes.

Thanks, Roger.