Re: [Xen-devel] [PATCH 3/5] hvm/mtrr: copy hardware state for Dom0

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 14.05.18 at 18:33, <roger.pau@xxxxxxxxxx> wrote:
> On Mon, May 14, 2018 at 08:26:30AM -0600, Jan Beulich wrote:
>> >>> On 10.05.18 at 19:15, <roger.pau@xxxxxxxxxx> wrote:
>> > Copy the state found on the hardware when creating a PVH Dom0. Since
>> > the memory map provided to a PVH Dom0 is based on the native one using
>> > the same set of MTRR ranges should provide Dom0 with a sane MTRR state
>> > without having to manually build it in Xen.
>> > 
>> > Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>> > ---
>> > Cc: Jan Beulich <jbeulich@xxxxxxxx>
>> > Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> > ---
>> >  xen/arch/x86/hvm/mtrr.c | 23 +++++++++++++++++++++++
>> >  1 file changed, 23 insertions(+)
>> > 
>> > diff --git a/xen/arch/x86/hvm/mtrr.c b/xen/arch/x86/hvm/mtrr.c
>> > index 95a3deabea..1cb000388a 100644
>> > --- a/xen/arch/x86/hvm/mtrr.c
>> > +++ b/xen/arch/x86/hvm/mtrr.c
>> > @@ -176,6 +176,29 @@ int hvm_vcpu_cacheattr_init(struct vcpu *v)
>> >          ((uint64_t)PAT_TYPE_UC_MINUS << 48) |       /* PAT6: UC- */
>> >          ((uint64_t)PAT_TYPE_UNCACHABLE << 56);      /* PAT7: UC */
>> >  
>> > +    if ( is_hardware_domain(v->domain) )
>> > +    {
>> > +        /* Copy values from the host. */
>> > +        struct domain *d = v->domain;
>> > +        unsigned int i;
>> > +
>> > +        if ( mtrr_state.have_fixed )
>> > +            for ( i = 0; i < NUM_FIXED_MSR; i++ )
>> > +                mtrr_fix_range_msr_set(d, m, i,
>> > +                                      ((uint64_t 
>> > *)mtrr_state.fixed_ranges)[i]);
>> 
>> The presence/absence of fixed range MTRRs needs to be reflected in the
>> capabilities MSR. Strictly speaking in their absence MSR access attempts to
>> the fixed range MSRs should also cause #GP, as should any attempt to
>> enable them in defType.
> 
> My intention was to always provide the fixed range MTRR capability,
> regardless of whether the underlying hardware has it or not. Then of
> course fixed ranges won't be enabled by default in the deftype MSR if
> the underlying hardware also hasn't got them enabled.

What would the result be of the OS writing to any of these MSRs, or
setting the respective enable bit?

Jan


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