Re: [Xen-devel][PATCH 02/02] VMX: Add HVM RDTSCP support

["Jan Beulich" <JBeulich@xxxxxxxxxx>]

>>> "Xu, Dongxiao" <dongxiao.xu@xxxxxxxxx> 11.12.09 12:53 >>>
>Add RDTSCP instruction support for HVM VMX guest.
> - RDTSCP is introduced in Nehalem processor on Intel platform. Like RDTSC,
>RDTSCP will return the TSC value, besides, it will return the low 32bit of
>TSC_AUX MSR. Currently Linux kernel will write (node_id << 12 | process_id)
>into that MSR, so that when guest execs RDTSCP, it will also get processor
>information.
> - This instruction is supported for HVM only when the hardware has this
>capability (indicated by cpuid).

This

>--- a/xen/arch/x86/hvm/vmx/vmx.c       Tue Dec 08 14:14:27 2009 +0000
>+++ b/xen/arch/x86/hvm/vmx/vmx.c       Fri Dec 11 19:10:53 2009 +0800
>@@ -138,7 +138,7 @@ static DEFINE_PER_CPU(struct vmx_msr_sta
> 
> static u32 msr_index[VMX_MSR_COUNT] =
> {
>-    MSR_LSTAR, MSR_STAR, MSR_SYSCALL_MASK
>+    MSR_LSTAR, MSR_STAR, MSR_SYSCALL_MASK, MSR_TSC_AUX
> };
> 
> static void vmx_save_host_msrs(void)
>@@ -146,8 +146,11 @@ static void vmx_save_host_msrs(void)
>     struct vmx_msr_state *host_msr_state = &this_cpu(host_msr_state);
>     int i;
> 
>-    for ( i = 0; i < VMX_MSR_COUNT; i++ )
>+    for ( i = 0; i < VMX_MSR_COUNT - 1; i++ )
>         rdmsrl(msr_index[i], host_msr_state->msrs[i]);
>+
>+    if ( cpu_has_rdtscp )
>+        rdmsrl(MSR_TSC_AUX, host_msr_state->msrs[VMX_INDEX_MSR_TSC_AUX]);
> }
> 
> #define WRITE_MSR(address)                                              \

seems to be calling for future problems - if anyone adds an element to
msr_index[] before MSR_TSC_AUX, the logic will silently break. A
BUILD_BUG_ON() and a comment would be the minimum thing to add
in my opinion.

Jan


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