Re: [PATCH 14/15] KVM: MTRR: do not map huage page for non-consistent range

[Paolo Bonzini]

On 04/06/2015 10:23, Xiao Guangrong wrote:
>>
>> So, why do you need to always use IPAT=0?  Can patch 15 keep the current
>> logic for RAM, like this:
>>
>>     if (is_mmio || kvm_arch_has_noncoherent_dma(vcpu->kvm))
>>         ret = kvm_mtrr_get_guest_memory_type(vcpu, gfn) <<
>>               VMX_EPT_MT_EPTE_SHIFT;
>>     else
>>         ret = (MTRR_TYPE_WRBACK << VMX_EPT_MT_EPTE_SHIFT)
>>             | VMX_EPT_IPAT_BIT;
> 
> Yeah, it's okay, actually we considered this way, however
> - it's light enough, it did not hurt guest performance based on our
>   benchmark.
> - the logic has always used for noncherent_dma case, extend it to
>   normal case should have low risk and also help us to check the logic.

But noncoherent_dma is not the common case, so it's not necessarily true
that the risk is low.

> - completely follow MTRRS spec would be better than host hides it.

We are a virtualization platform, we know well when MTRRs are necessary.

Tis a risk from blindly obeying the guest MTRRs: userspace can see stale
data if the guest's accesses bypass the cache.  AMD bypasses this by
enabling snooping even in cases that ordinarily wouldn't snoop; for
Intel the solution is that RAM-backed areas should always use IPAT.

Paolo
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/