Re: xen 4.15.5: msr_relaxed required for MSR 0x1a2

[James Dingwall <james@xxxxxxxxxxxxxx>]

On Thu, Nov 16, 2023 at 04:32:47PM +0000, Andrew Cooper wrote:
> On 16/11/2023 4:15 pm, James Dingwall wrote:
> > Hi,
> >
> > Per the msr_relaxed documentation:
> >
> >    "If using this option is necessary to fix an issue, please report a bug."
> >
> > After recently upgrading an environment from Xen 4.14.5 to Xen 4.15.5 we
> > started experiencing a BSOD at boot with one of our Windows guests.  We 
> > found
> > that enabling `msr_relaxed = 1` in the guest configuration has resolved the
> > problem.  With a debug build of Xen and `hvm_debug=2048` on the command line
> > the following messages were caught as the BSOD happened:
> >
> > (XEN) [HVM:11.0] <vmx_msr_read_intercept> ecx=0x1a2
> > (XEN) vmx.c:3298:d11v0 RDMSR 0x000001a2 unimplemented
> > (XEN) d11v0 VIRIDIAN CRASH: 1e ffffffffc0000096 fffff80b8de81eb5 0 0
> >
> > I found that MSR 0x1a2 is MSR_TEMPERATURE_TARGET and from that this patch
> > series from last month:
> >
> > https://patchwork.kernel.org/project/xen-devel/list/?series=796550
> >
> > Picking out just a small part of that fixes the problem for us. Although the
> > the patch is against 4.15.5 I think it would be relevant to more recent
> > releases too.
> 
> Which version of Windows, and what hardware?
> 
> The Viridian Crash isn't about the RDMSR itself - it's presumably
> collateral damage shortly thereafter.
> 
> Does filling in 0 for that MSR also resolve the issue?  It's model
> specific and we absolutely cannot pass it through from real hardware
> like that.
> 

Hi Andrew,

Thanks for your response.  The guest is running Windows 10 and the crash
happens in a proprietary hardware driver.  A little bit of knowledge as
they say was enough to stop the crash but I don't understand the impact
of what I've actually done...

To rework the patch I'd need a bit of guidance, if I understand your
suggestion I set the MSR to 0 with this change in emul-priv-op.c:

diff --git a/xen/arch/x86/pv/emul-priv-op.c b/xen/arch/x86/pv/emul-priv-op.c
index ed97b1d6fcc..66f5e417df6 100644
--- a/xen/arch/x86/pv/emul-priv-op.c
+++ b/xen/arch/x86/pv/emul-priv-op.c
@@ -976,6 +976,10 @@ static int read_msr(unsigned int reg, uint64_t *val,
         *val = 0;
         return X86EMUL_OKAY;
 
+    case MSR_TEMPERATURE_TARGET:
+        *val = 0;
+        return X86EMUL_OKAY;
+
     case MSR_P6_PERFCTR(0) ... MSR_P6_PERFCTR(7):
     case MSR_P6_EVNTSEL(0) ... MSR_P6_EVNTSEL(3):
     case MSR_CORE_PERF_FIXED_CTR0 ... MSR_CORE_PERF_FIXED_CTR2:

and this in vmx.c:

diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
index 54023a92587..bbf37b7f272 100644
--- a/xen/arch/x86/hvm/vmx/vmx.c
+++ b/xen/arch/x86/hvm/vmx/vmx.c
@@ -3259,6 +3259,11 @@ static int vmx_msr_read_intercept(unsigned int msr, 
uint64_t *msr_content)
         if ( !nvmx_msr_read_intercept(msr, msr_content) )
             goto gp_fault;
         break;
+
+    case MSR_TEMPERATURE_TARGET:
+        *msr_content = 0;
+        break;
+
     case MSR_IA32_MISC_ENABLE:
         rdmsrl(MSR_IA32_MISC_ENABLE, *msr_content);
         /* Debug Trace Store is not supported. */


Thanks,
James