Re: [Xen-devel] [PATCH for-4.8] libxc/x86: Report consistent initial APIC value for PV guests

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 10/11/16 14:50, Boris Ostrovsky wrote:
> Currently hypervisor provides PV guest's CPUID(1).EBX[31:24] (initial
> APIC ID) with contents of that field on the processor that launched
> the guest. This results in the guest reporting different initial
> APIC IDs across runs.
>
> We should be consistent in how this value is reported, let's set
> it to 0 (which is also what Linux guests expect).
>
> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>

This surely wants to go along with:

andrewcoop@andrewcoop:/local/xen.git/xen$ git diff
diff --git a/xen/arch/x86/traps.c b/xen/arch/x86/traps.c
index b51b51b..bdf9339 100644
--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -985,6 +985,10 @@ void pv_cpuid(struct cpu_user_regs *regs)
         uint32_t tmp, _ecx, _ebx;
 
     case 0x00000001:
+        /* Fix up VLAPIC details. */
+        b &= 0x00FFFFFFu;
+        b |= (curr->vcpu_id * 2) << 24;
+
         c &= pv_featureset[FEATURESET_1c];
         d &= pv_featureset[FEATURESET_1d];
 

Which brings the PV CPUID handling in line with HVM handling.  Otherwise
a guest will see an APIC ID of 0 in all vcpus, which will surely confuse it.

~Andrew

> ---
>
> I think this should go to stable branches as well. This has been causing
> problems lately in Linux with introduction of topology maps.
>
>
>  tools/libxc/xc_cpuid_x86.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/tools/libxc/xc_cpuid_x86.c b/tools/libxc/xc_cpuid_x86.c
> index d761805..1f26294 100644
> --- a/tools/libxc/xc_cpuid_x86.c
> +++ b/tools/libxc/xc_cpuid_x86.c
> @@ -618,6 +618,12 @@ static void xc_cpuid_pv_policy(xc_interface *xch,
>          /* Host topology exposed to PV guest.  Provide host value. */
>          bool host_htt = regs[3] & bitmaskof(X86_FEATURE_HTT);
>  
> +        /*
> +         * Don't pick host's Initial APIC ID which can change from run
> +         * to run. 
> +         */
> +        regs[1] &= 0x00ffffffu;
> +
>          regs[2] = info->featureset[featureword_of(X86_FEATURE_SSE3)];
>          regs[3] = (info->featureset[featureword_of(X86_FEATURE_FPU)] &
>                     ~bitmaskof(X86_FEATURE_HTT));


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