[Qemu-devel] Re: [RFC PATCH 5/5] VFIO based device assignment

[Avi Kivity <avi@redhat.com>]

On 07/11/2010 10:38 PM, Alex Williamson wrote:
>
>> What
>> about page attributes?
>>
>> There are two cases:
>>
>> - snoop capable iommu - can use write-backed RAM, but need to enable
>> snoop.  BARs still need to respect page attributes.
>> - older mmu - need to respect guest memory type; probably cannot be done
>> without kvm.
>>
>> If the guest maps a BAR or RAM using write-combine memory type, can we
>> reflect that?  This may provide a considerable performance benefit.
>>      
> Do we do anything about this today in kvm device assignment?  Maybe it's
> buried in the kernel side bits and I've missed it.  I would expect that
> WC mappings in the guest carry through to host virtual mappings, but
> maybe we can only do that with kvm.

Yes, see arch/x86/kvm/mmu.c, set_spte() calling ->get_mt_mask().

Strangely, it's qualified with tdp.  Perhaps because of all of the scary 
errata regarding mismatching memory types for a page.

> The processor side mappings are
> independent of the iommu mappings since devices don't care about such
> things.  Thanks,
>    

Yeah.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.