Subject: Re: [PATCH] KVM: allow NULL writable argument to __kvm_faultin_pfn

From: Sean Christopherson <[email-masked]>
Date: Mon, 6 Jan 2025 10:57:53 -0800

On Wed, Jan 01, 2025, Paolo Bonzini wrote:
> kvm_follow_pfn() is able to work with NULL in the .map_writable field
> of the homonymous struct.  But __kvm_faultin_pfn() rejects the combo
> despite KVM for e500 trying to use it.  Indeed .map_writable is not
> particularly useful if the flags include FOLL_WRITE and readonly
> guest memory is not supported, so add support to __kvm_faultin_pfn()
> for this case.

I would prefer to keep the sanity check to minimize the risk of a page fault
handler not supporting opportunistic write mappings.  e500 is definitely the
odd one out here.

What about adding a dedicated wrapper for getting a writable PFN?  E.g. (untested)

---
 arch/powerpc/kvm/e500_mmu_host.c | 2 +-
 arch/x86/kvm/vmx/vmx.c           | 3 +--
 include/linux/kvm_host.h         | 8 ++++++++
 3 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/arch/powerpc/kvm/e500_mmu_host.c b/arch/powerpc/kvm/e500_mmu_host.c
index e5a145b578a4..2251bb30b8ec 100644
--- a/arch/powerpc/kvm/e500_mmu_host.c
+++ b/arch/powerpc/kvm/e500_mmu_host.c
@@ -444,7 +444,7 @@ static inline int kvmppc_e500_shadow_map(struct kvmppc_vcpu_e500 *vcpu_e500,
 
 	if (likely(!pfnmap)) {
 		tsize_pages = 1UL << (tsize + 10 - PAGE_SHIFT);
-		pfn = __kvm_faultin_pfn(slot, gfn, FOLL_WRITE, NULL, &page);
+		pfn = kvm_faultin_writable_pfn(slot, gfn, &page);
 		if (is_error_noslot_pfn(pfn)) {
 			if (printk_ratelimit())
 				pr_err("%s: real page not found for gfn %lx\n",
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 893366e53732..7012b583f2e8 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6800,7 +6800,6 @@ void vmx_set_apic_access_page_addr(struct kvm_vcpu *vcpu)
 	struct page *refcounted_page;
 	unsigned long mmu_seq;
 	kvm_pfn_t pfn;
-	bool writable;
 
 	/* Defer reload until vmcs01 is the current VMCS. */
 	if (is_guest_mode(vcpu)) {
@@ -6836,7 +6835,7 @@ void vmx_set_apic_access_page_addr(struct kvm_vcpu *vcpu)
 	 * controls the APIC-access page memslot, and only deletes the memslot
 	 * if APICv is permanently inhibited, i.e. the memslot won't reappear.
 	 */
-	pfn = __kvm_faultin_pfn(slot, gfn, FOLL_WRITE, &writable, &refcounted_page);
+	pfn = kvm_faultin_writable_pfn(slot, gfn, &refcounted_page);
 	if (is_error_noslot_pfn(pfn))
 		return;
 
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index c788d0bd952a..b0af7c7f99da 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -1287,6 +1287,14 @@ static inline kvm_pfn_t kvm_faultin_pfn(struct kvm_vcpu *vcpu, gfn_t gfn,
 				 write ? FOLL_WRITE : 0, writable, refcounted_page);
 }
 
+static inline kvm_pfn_t kvm_faultin_writable_pfn(const struct kvm_memory_slot *slot,
+						 gfn_t gfn, struct page **refcounted_page)
+{
+	bool writable;
+
+	return __kvm_faultin_pfn(slot, gfn, FOLL_WRITE, &writable, refcounted_page);
+}
+
 int kvm_read_guest_page(struct kvm *kvm, gfn_t gfn, void *data, int offset,
 			int len);
 int kvm_read_guest(struct kvm *kvm, gpa_t gpa, void *data, unsigned long len);

base-commit: 2c3412e999738bfd60859c493ff47f5c268814a3
--