Re: [Xen-devel] [PATCH] Limit MCG Cap

[David Lively <dlively@xxxxxxxxxxxxxxx>]

I don't see why not, now that you mention it.

I'll give it a try ...

Dave


Keir Fraser wrote:
> Given that we don't properly virtualise/emulate machine check (we only set
> the feature bit because some versions of Windows demand it) can we get away
> with returning zero for reads of MCG_CAP?
>
>  -- Keir
>
> On 27/8/07 20:11, "Ben Guthro" <bguthro@xxxxxxxxxxxxxxx> wrote:
>
>   
> > Intercept guest reads of MSR_IA32_MCG_CAP and limit the number of memory banks
> > reported to one.
> > This prevents us from trying to read status of non-existent banks when
> > migrated to a machine
> > with fewer banks.
> >
> > Signed-off-by: Ben Guthro
> > Signed-off-by: David Lively <dlively@xxxxxxxxxxxxxxx>
> >
> > diff -r 4eea7c9b3bf2 xen/arch/x86/hvm/svm/svm.c
> > --- a/xen/arch/x86/hvm/svm/svm.c Thu Aug 02 08:59:43 2007 -0400
> > +++ b/xen/arch/x86/hvm/svm/svm.c Thu Aug 02 08:59:43 2007 -0400
> > @@ -2162,7 +2162,19 @@ static void svm_do_msr_access(
> >              /* No point in letting the guest see real MCEs */
> >              msr_content = 0;
> >              break;
> > -
> > +        case MSR_IA32_MCG_CAP:
> > +            if ( rdmsr_safe(ecx, regs->eax, regs->edx) == 0 ) {
> > +                /*
> > +                 * Report at most one memory bank so migration to a machine
> > with
> > +                 * fewer banks doesn't cause guest GPFs when the guest tries
> > to read the
> > +                 * (now, after migration) non-existent banks' MSRs.
> > +                 */
> > +                regs->eax = (regs->eax & ~0xFF) | !!(regs->eax & 0xFF);
> > +                goto done;
> > +            }
> > +            gdprintk(XENLOG_ERR, "%s: rdmsr_safe(MCG_CAP) failed!\n",
> > __FUNCTION__);
> > +            svm_inject_exception(v, TRAP_gp_fault, 1, 0);
> > +            return;
> >          default:
> >              if ( rdmsr_hypervisor_regs(ecx, &eax, &edx) ||
> >                   rdmsr_safe(ecx, eax, edx) == 0 )
> > diff -r 4eea7c9b3bf2 xen/arch/x86/hvm/vmx/vmx.c
> > --- a/xen/arch/x86/hvm/vmx/vmx.c Thu Aug 02 08:59:43 2007 -0400
> > +++ b/xen/arch/x86/hvm/vmx/vmx.c Thu Aug 02 09:14:06 2007 -0400
> > @@ -2754,6 +2754,19 @@ static int vmx_do_msr_read(struct cpu_us
> >          /* No point in letting the guest see real MCEs */
> >          msr_content = 0;
> >          break;
> > +    case MSR_IA32_MCG_CAP:
> > +        if ( rdmsr_safe(ecx, regs->eax, regs->edx) == 0 ) {
> > +            /*
> > +             * Report only a single memory bank so migration to a machine
> > with
> > +             * fewer banks doesn't cause guest GPFs when the guest tries to
> > read the
> > +             * (now, after migration) non-existent banks' MSRs.
> > +             */
> > +            regs->eax = (regs->eax & ~0xFF) | !!(regs->eax & 0xFF);
> > +            goto done;
> > +        }
> > +        gdprintk(XENLOG_ERR, "%s: rdmsr_safe(MCG_CAP) failed!\n",
> > __FUNCTION__);
> > +        vmx_inject_hw_exception(v, TRAP_gp_fault, 0);
> > +        return 0;
> >      default:
> >          switch ( long_mode_do_msr_read(regs) )
> >          {
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@xxxxxxxxxxxxxxxxxxx
> > http://lists.xensource.com/xen-devel
> >     
>   

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