linux-kernel - Re: [PATCH v2 1/6] KVM: x86/mmu: add a new mmu zap helper to indicate memtype changes

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Date:   Tue, 30 May 2023 17:48:10 +0800
From:   Yan Zhao <yan.y.zhao@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>
CC:     Chao Gao <chao.gao@...el.com>, <kvm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <pbonzini@...hat.com>
Subject: Re: [PATCH v2 1/6] KVM: x86/mmu: add a new mmu zap helper to
 indicate memtype changes

On Thu, May 25, 2023 at 08:54:15AM -0700, Sean Christopherson wrote:
> > > static void kvm_noncoherent_dma_start_or_end(struct kvm *kvm)
> > What does start_or_end or first_or_last stand for? 
> 
> Start/End of device (un)assignment, or First/Last device (un)assigned.  Definitely
> feel free to pick a better name.
Hi Sean,
start_or_end gave me a first impression that it stands for gfn_start, gfn_end.
so currently I changed it to start/stop to avoid confusion.

+static void kvm_noncoherent_dma_assignment_start_or_stop(struct kvm *kvm)
+{
+       /*
+        * Only necessary to zap KVM TDP if guest MTRR is honored.
+        * As whether a VM has noncoherent DMA can affect whether
+        * KVM honors guest MTRR and the resulting memory type in TDP,
+        * specify its value as true here to test if guest MTRR
+        * is honored after the assignment starts or
+        * was honored before the assignment stops.
+        */
+       if (kvm_mmu_honors_guest_mtrrs(kvm, true))
+               kvm_zap_gfn_range(kvm, 0, ~0ULL);
+}

And I combined the __kvm_mmu_honors_guest_mtrrs() into
kvm_mmu_honors_guest_mtrrs(). Not sure if you like it :)

+/*
+ * Returns if KVM honors guest MTRRs
+ * @override_vm_has_noncoherent_dma: Allow caller to override non-coherent DMA
+ *                                   status returned from
+ *                                   kvm_arch_has_noncoherent_dma()
+ */
+bool kvm_mmu_honors_guest_mtrrs(struct kvm *kvm,
+                               bool override_vm_has_noncoherent_dma)
+{
+       bool noncoherent_dma = override_vm_has_noncoherent_dma ? true :
+                              kvm_arch_has_noncoherent_dma(kvm);
+
+       /*
+        * If the TDP is enabled, the host MTRRs are ignored by TDP
+        * (shadow_memtype_mask is non-zero), and the VM has non-coherent DMA
+        * (DMA doesn't snoop CPU caches), KVM's ABI is to honor the memtype
+        * from the guest's MTRRs so that guest accesses to memory that is
+        * DMA'd aren't cached against the guest's wishes.
+        *
+        * Note, KVM may still ultimately ignore guest MTRRs for certain PFNs,
+        * e.g. KVM will force UC memtype for host MMIO.
+        */
+       return noncoherent_dma && tdp_enabled && shadow_memtype_mask;
+}
+

Thanks
Yan