Re: [Xen-devel] [PATCH 2/3] x86/msr: Drop {MISC_ENABLES, PLATFORM_INFO}.available

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 02.07.18 at 11:57, <andrew.cooper3@xxxxxxxxxx> wrote:
> These MSRs are non-architectural and the available booleans were used in lieu
> of an architectural signal of availability.  The MSRs are unconditionally
> available to HVM guests, but currently for PV guests, are hidden when CPUID
> faulting is unavailable.
> 
> However, in hindsight, the additional booleans make toolstack MSR interactions
> more complicated.  As the behaviour of the MSRs is reserved when unavailable,

Is it? Isn't the expected (mandated?) behavior raising of #GP(0) in that
case?

> unconditionally letting the MSRs be accessible is compatible behaviour, even
> for PV guests.
> 
> The new behaviour is:
>   * PLATFORM_INFO is unconditionally readable even for PV guests and will
>     indicate the presense or absense of CPUID Faulting in bit 31.
>   * MISC_FEATURES_ENABLES is uncondtionally readable, and bit 0 may be set iff
>     PLATFORM_INFO reports that CPUID Faulting is available.

For these two I agree with the intended new behavior though (despite
seeing a tiny bit of a risk); I merely think the statement above is a little
too broad.

> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

Jan



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