Re: [RFC PATCH] KVM: Introduce KVM VIRTIO device

[Yan Zhao]

On Mon, Dec 18, 2023 at 07:08:51AM -0800, Sean Christopherson wrote:
> > > Implementation Consideration
> > > ===
> > > There is a previous series [1] from google to serve the same purpose to
> > > let KVM be aware of virtio GPU's noncoherent DMA status. That series
> > > requires a new memslot flag, and special memslots in user space.
> > > 
> > > We don't choose to use memslot flag to request honoring guest memory
> > > type.
> > 
> > memslot flag has the potential to restrict the impact e.g. when using
> > clflush-before-read in migration?
> 
> Yep, exactly.  E.g. if KVM needs to ensure coherency when freeing memory back to
> the host kernel, then the memslot flag will allow for a much more targeted
> operation.
> 
> > Of course the implication is to honor guest type only for the selected slot
> > in KVM instead of applying to the entire guest memory as in previous series
> > (which selects this way because vmx_get_mt_mask() is in perf-critical path
> > hence not good to check memslot flag?)
> 
> Checking a memslot flag won't impact performance.  KVM already has the memslot
> when creating SPTEs, e.g. the sole caller of vmx_get_mt_mask(), make_spte(), has
> access to the memslot.
> 
> That isn't coincidental, KVM _must_ have the memslot to construct the SPTE, e.g.
> to retrieve the associated PFN, update write-tracking for shadow pages, etc.
> 
Hi Sean,
Do you prefer to introduce a memslot flag KVM_MEM_DMA or KVM_MEM_WC?
For KVM_MEM_DMA, KVM needs to
(a) search VMA for vma->vm_page_prot and convert it to page cache mode (with
    pgprot2cachemode()? ), or
(b) look up memtype of the PFN, by calling lookup_memtype(), similar to that in
    pat_pfn_immune_to_uc_mtrr().

But pgprot2cachemode() and lookup_memtype() are not exported by x86 code now.

For KVM_MEM_WC, it requires user to ensure the memory is actually mapped
to WC, right?

Then, vmx_get_mt_mask() just ignores guest PAT and programs host PAT as EPT type
for the special memslot only, as below.
Is this understanding correct?

static u8 vmx_get_mt_mask(struct kvm_vcpu *vcpu, gfn_t gfn, bool is_mmio)
{
        if (is_mmio)                                                                           
                return MTRR_TYPE_UNCACHABLE << VMX_EPT_MT_EPTE_SHIFT;                          
                                                                                               
        if (gfn_in_dma_slot(vcpu->kvm, gfn)) {                                                 
                u8 type = MTRR_TYPE_WRCOMB;                                      
                //u8 type = pat_pfn_memtype(pfn);                                
                return (type << VMX_EPT_MT_EPTE_SHIFT) | VMX_EPT_IPAT_BIT;       
        }                                                                                      
                                                                                               
        if (!kvm_arch_has_noncoherent_dma(vcpu->kvm))                            
                return (MTRR_TYPE_WRBACK << VMX_EPT_MT_EPTE_SHIFT) | VMX_EPT_IPAT_BIT;         
                                                                                               
        if (kvm_read_cr0_bits(vcpu, X86_CR0_CD)) {                                             
                if (kvm_check_has_quirk(vcpu->kvm, KVM_X86_QUIRK_CD_NW_CLEARED))               
                        return MTRR_TYPE_WRBACK << VMX_EPT_MT_EPTE_SHIFT;                      
                else                                                                           
                        return (MTRR_TYPE_UNCACHABLE << VMX_EPT_MT_EPTE_SHIFT) | 
                                VMX_EPT_IPAT_BIT;                                
        }                                                                        
                                                                                 
        return kvm_mtrr_get_guest_memory_type(vcpu, gfn) << VMX_EPT_MT_EPTE_SHIFT;
}

BTW, since the special memslot must be exposed to guest as virtio GPU BAR in
order to prevent other guest drivers from access, I wonder if it's better to
include some keyword like VIRTIO_GPU_BAR in memslot flag name.