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

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Date:	Thu, 04 Jun 2015 10:36:47 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Xiao Guangrong <guangrong.xiao@...ux.intel.com>
CC:	gleb@...nel.org, mtosatti@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 14/15] KVM: MTRR: do not map huage page for non-consistent
 range



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
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