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Re: [Xen-devel] PVH and mtrr/PAT.........

On 11/20/2013 9:42 PM, Mukesh Rathor wrote:> On Wed, 20 Nov 2013 08:42:08 +0000
> "Jan Beulich" <JBeulich@xxxxxxxx> wrote:
>>>>> On 20.11.13 at 03:11, Mukesh Rathor <mukesh.rathor@xxxxxxxxxx>
>>>>> wrote:
>>> After rebasing my dom0 on latest, it didn't boot. After debugging
>>> couple days, it turned out to be :
>>> +    if ( is_pvh_domain(d) )
>>> +    {
>>> +        if ( direct_mmio )
>>> +            return MTRR_TYPE_UNCACHABLE;
>>> +        return MTRR_TYPE_WRBACK;
>>> +    }
>>> +
>>> I had in my patches, missing in epte_get_entry_emt() in latest.
>>> So, since I don't know much about this, is an HVM guest setting
>>> MTRR range types? Looking for suggestions on best way to do this
>>> for PVH.
>> A HVM guest is permitted to write to (virtual) MTRRs, whereas a PV
>> guest isn't. I'm inclined to prefer PV behavior here to be used for
>> PVH (since, as explained by Dongxiao, MTRRs don't really matter
>> for VMX guests anyway, i.e. the setting of (virtual) MTRRs needs to
>> get translated to EPT memory types anyway, hence a PVH guest
>> ought to be fine ignoring the MTRRs altogether and handling memory
>> types exclusively via PAT mechanisms).
> Ok. So, it appears that for PV, we store the cacheattr
> in page_info and use it during pte update. But in case of PVH,
> the page tables are native, the pte update is native, so we
> don't really have access to PCD/PWT/PAT bits in the pte entry!

Right, which is OK - b/c the mechanism to set a WC page and back
is at odds with how the Linux sets its bits. This is a problem
for PV guests (pvops based) as they cannot do PAT right now.

HVM guest can as they omit all of this.
> It says PAT+PWT+PCD selects a PAT entry from the IA32_PAT msr.

> In case of PVH, the msr is guest managed, and intercept is disabled.

I thought the IA32_PAT MSR was intercepted?

> I assume the EPT should mirror the pte PAT entries?
> Or, can we just set WB for all RAM, and UC for all non-ram for
> PVH and keep it simple?

If you are using an i915 it will want to map its MMIO bars as WC.
Ditto for InfiniBand cards, radeon and nouveau cards.

> Thanks a lot for the help.
> Mukesh
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