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 kvm" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
