Re: [PATCH] x86/svm: ignore accesses to EX_CFG

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, Sep 16, 2020 at 02:55:52PM +0200, Jan Beulich wrote:
> On 16.09.2020 12:54, Roger Pau Monne wrote:
> > Windows 10 will try to unconditionally read EX_CFG on AMD hadrware,
> > and injecting a #GP fault will result in a panic:
> > 
> > svm.c:1964:d5v0 RDMSR 0xc001102c unimplemented
> > d5v0 VIRIDIAN CRASH: 7e ffffffffc0000096 fffff8054cbe5ffe fffffa0837a066e8 
> > fffffa0837a05f30
> > 
> > Return 0 when trying to read the MSR and drop writes.
> 
> So I've gone through a bunch of BKDGs and PPRs, without finding
> this MSR mentioned in any of them. Could you point out on which
> model(s) it actually exists? You must have found it somewhere,
> or else you wouldn't know a name for it...

Yes, sorry it took me a while to find it also, and I should have added
a reference here. It's in "BIOS and Kernel Developer’s Guide (BKDG)
for AMD Family 15h Models 00h-0Fh Processors", albeit Windows will try
to access it on Family 17h also.

> > @@ -2108,6 +2109,7 @@ static int svm_msr_write_intercept(unsigned int msr, 
> > uint64_t msr_content)
> >      case MSR_K8_TOP_MEM2:
> >      case MSR_K8_SYSCFG:
> >      case MSR_K8_VM_CR:
> > +    case MSR_AMD64_EX_CFG:
> >          /* ignore write. handle all bits as read-only. */
> >          break;
> 
> Is this necessary, rather than having writes fault?

Hm, I'm not sure about that. This is the same that KVM did to handle
the MSR, see Linux commit 0e1b869fff60c81b510c2d00602d778f8f59dd9a.

I can try to return #GP for writes, but I don't see much issue in just
ignoring writes.

> > --- a/xen/include/asm-x86/msr-index.h
> > +++ b/xen/include/asm-x86/msr-index.h
> > @@ -330,6 +330,7 @@
> >  #define MSR_AMD64_DC_CFG           0xc0011022
> >  #define MSR_AMD64_DE_CFG           0xc0011029
> >  #define AMD64_DE_CFG_LFENCE_SERIALISE      (_AC(1, ULL) << 1)
> > +#define MSR_AMD64_EX_CFG                0xc001102c
> 
> Indentation here wants to match the siblings, i.e. use hard tabs
> (for now). Easily addressed while committing, of course.

Oh, sure, sorry for not realizing.

Thanks, Roger.