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

[Sean Christopherson]

On Wed, Dec 20, 2023, Yan Zhao wrote:
> On Tue, Dec 19, 2023 at 12:26:45PM +0800, Yan Zhao wrote:
> > 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.
> Another choice is to add a memslot flag KVM_MEM_HONOR_GUEST_PAT, then user
> (e.g. QEMU) does special treatment to this kind of memslots (e.g. skipping
> reading/writing to them in general paths).
> 
> @@ -7589,26 +7589,29 @@ 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 (in_slot_honor_guest_pat(vcpu->kvm, gfn))
> +               return kvm_mtrr_get_guest_memory_type(vcpu, gfn) << VMX_EPT_MT_EPTE_SHIFT;

This is more along the lines of what I was thinking, though the name should be
something like KVM_MEM_NON_COHERENT_DMA, i.e. not x86 specific and not contradictory
for AMD (which already honors guest PAT).

I also vote to deliberately ignore MTRRs, i.e. start us on the path of ripping
those out.  This is a new feature, so we have the luxury of defining KVM's ABI
for that feature, i.e. can state that on x86 it honors guest PAT, but not MTRRs.

Like so?

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index d21f55f323ea..ed527acb2bd3 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -7575,7 +7575,8 @@ static int vmx_vm_init(struct kvm *kvm)
        return 0;
 }
 
-static u8 vmx_get_mt_mask(struct kvm_vcpu *vcpu, gfn_t gfn, bool is_mmio)
+static u8 vmx_get_mt_mask(struct kvm_vcpu *vcpu, gfn_t gfn, bool is_mmio,
+                         struct kvm_memory_slot *slot)
 {
        /* We wanted to honor guest CD/MTRR/PAT, but doing so could result in
         * memory aliases with conflicting memory types and sometimes MCEs.
@@ -7598,6 +7599,9 @@ 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 (kvm_memslot_has_non_coherent_dma(slot))
+               return MTRR_TYPE_WRBACK << VMX_EPT_MT_EPTE_SHIFT;
+
        if (!kvm_arch_has_noncoherent_dma(vcpu->kvm))
                return (MTRR_TYPE_WRBACK << VMX_EPT_MT_EPTE_SHIFT) | VMX_EPT_IPAT_BIT;

I like the idea of pulling the memtype from the host, but if we can make that
work then I don't see the need for a special memslot flag, i.e. just do it for
*all* SPTEs on VMX.  I don't think we need a VMA for that, e.g. we should be able
to get the memtype from the host PTEs, just like we do the page size.

KVM_MEM_WC is a hard "no" for me.  It's far too x86 centric, and as you alluded
to, it requires coordination from the guest, i.e. is effectively limited to
paravirt scenarios.

> +
>         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;
>  }