Re: [PATCH v12 45/84] KVM: guest_memfd: Provide "struct page" as output from kvm_gmem_get_pfn()

Paolo Bonzini
2024-07-30T09:06:18Z

On 7/27/24 01:51, Sean Christopherson wrote:
> Provide the "struct page" associated with a guest_memfd pfn as an output
> from __kvm_gmem_get_pfn() so that KVM guest page fault handlers can
^^^^^^^^^^^^^^^^^^^^

Just "kvm_gmem_get_pfn()".

> directly put the page instead of having to rely on
> kvm_pfn_to_refcounted_page().

This will conflict with my series, where I'm introducing
folio_file_pfn() and using it here:
> - page = folio_file_page(folio, index);
> + *page = folio_file_page(folio, index);
>
> - *pfn = page_to_pfn(page);
> + *pfn = page_to_pfn(*page);
> if (max_order)
> *max_order = 0;

That said, I think it's better to turn kvm_gmem_get_pfn() into
kvm_gmem_get_page() here, and pull the page_to_pfn() or page_to_phys()
to the caller as applicable. This highlights that the caller always
gets a refcounted page with guest_memfd.

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 901be9e420a4..bcc4a4c594ef 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -4348,13 +4348,14 @@ static int kvm_faultin_pfn_private(struct kvm_vcpu *vcpu,
return -EFAULT;
}

- r = kvm_gmem_get_pfn(vcpu->kvm, fault->slot, fault->gfn, &fault->pfn,
+ r = kvm_gmem_get_page(vcpu->kvm, fault->slot, fault->gfn, &fault->refcounted_page,
&max_order);
if (r) {
kvm_mmu_prepare_memory_fault_exit(vcpu, fault);
return r;
}

+ fault->pfn = page_to_pfn(page);
fault->map_writable = !(fault->slot->flags & KVM_MEM_READONLY);
fault->max_level = kvm_max_private_mapping_level(vcpu->kvm, fault->pfn,
fault->max_level, max_order);
diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index a16c873b3232..db4181d11f2e 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -3847,7 +3847,7 @@ static int __sev_snp_update_protected_guest_state(struct kvm_vcpu *vcpu)
if (VALID_PAGE(svm->sev_es.snp_vmsa_gpa)) {
gfn_t gfn = gpa_to_gfn(svm->sev_es.snp_vmsa_gpa);
struct kvm_memory_slot *slot;
- kvm_pfn_t pfn;
+ struct page *page;

slot = gfn_to_memslot(vcpu->kvm, gfn);
if (!slot)
@@ -3857,7 +3857,7 @@ static int __sev_snp_update_protected_guest_state(struct kvm_vcpu *vcpu)
* The new VMSA will be private memory guest memory, so
* retrieve the PFN from the gmem backend.
*/
- if (kvm_gmem_get_pfn(vcpu->kvm, slot, gfn, &pfn, NULL))
+ if (kvm_gmem_get_page(vcpu->kvm, slot, gfn, &page, NULL))
return -EINVAL;

/*
@@ -3873,7 +3873,7 @@ static int __sev_snp_update_protected_guest_state(struct kvm_vcpu *vcpu)
svm->sev_es.snp_has_guest_vmsa = true;

/* Use the new VMSA */
- svm->vmcb->control.vmsa_pa = pfn_to_hpa(pfn);
+ svm->vmcb->control.vmsa_pa = page_to_phys(page);

/* Mark the vCPU as runnable */
vcpu->arch.pv.pv_unhalted = false;
@@ -3886,7 +3886,7 @@ static int __sev_snp_update_protected_guest_state(struct kvm_vcpu *vcpu)
* changes then care should be taken to ensure
* svm->sev_es.vmsa is pinned through some other means.
*/
- kvm_release_pfn_clean(pfn);
+ kvm_release_page_clean(page);
}

/*
@@ -4687,6 +4687,7 @@ void sev_handle_rmp_fault(struct kvm_vcpu *vcpu, gpa_t gpa, u64 error_code)
struct kvm *kvm = vcpu->kvm;
int order, rmp_level, ret;
bool assigned;
+ struct page *page;
kvm_pfn_t pfn;
gfn_t gfn;

@@ -4712,13 +4713,14 @@ void sev_handle_rmp_fault(struct kvm_vcpu *vcpu, gpa_t gpa, u64 error_code)
return;
}

- ret = kvm_gmem_get_pfn(kvm, slot, gfn, &pfn, &order);
+ ret = kvm_gmem_get_page(kvm, slot, gfn, &page, &order);
if (ret) {
pr_warn_ratelimited("SEV: Unexpected RMP fault, no backing page for private GPA 0x%llx\n",
gpa);
return;
}

+ pfn = page_to_pfn(page);
ret = snp_lookup_rmpentry(pfn, &assigned, &rmp_level);
if (ret || !assigned) {
pr_warn_ratelimited("SEV: Unexpected RMP fault, no assigned RMP entry found for GPA 0x%llx PFN 0x%llx error %d\n",
@@ -4770,7 +4772,7 @@ void sev_handle_rmp_fault(struct kvm_vcpu *vcpu, gpa_t gpa, u64 error_code)
out:
trace_kvm_rmp_fault(vcpu, gpa, pfn, error_code, rmp_level, ret);
out_no_trace:
- put_page(pfn_to_page(pfn));
+ kvm_release_page_unused(page);
}

static bool is_pfn_range_shared(kvm_pfn_t start, kvm_pfn_t end)

And the change in virt/kvm/guest_memfd.c then is just as trivial, apart
from all the renaming:

- *pfn = folio_file_pfn(folio, index);
+ *page = folio_file_page(folio, index);

Paolo