[PATCH v4 3/6] KVM: x86/xen: Explicitly tag "shared info" page as never being dirty tracked

[David Woodhouse]

From: Sean Christopherson <sea...@google.com>

Explicitly mark the Xen shared info page as never being dirty tracked so
that higher-level gpc APIs can be added to automatically take care of
things like dirty tracking, without reintroducing the bug fixed by commit
55749769fe60 ("KVM: x86: Fix wall clock writes in Xen shared_info not to
mark page dirty"). And because the code _looks_ buggy.

No functional change intended.

Signed-off-by: Sean Christopherson <sea...@google.com>
Signed-off-by: David Woodhouse <dw...@amazon.co.uk>
Assisted-by: Claude:claude-mythos-5
---
arch/x86/kvm/xen.c | 2 +-
include/linux/kvm_host.h | 10 ++++++++--
include/linux/kvm_types.h | 1 +
virt/kvm/pfncache.c | 4 +++-
4 files changed, 13 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
index 40cb56621e25..fab215a522be 100644
--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -2396,7 +2396,7 @@ void kvm_xen_init_vm(struct kvm *kvm)
{
mutex_init(&kvm->arch.xen.xen_lock);
idr_init(&kvm->arch.xen.evtchn_ports);
- kvm_gpc_init(&kvm->arch.xen.shinfo_cache, kvm);
+ __kvm_gpc_init(&kvm->arch.xen.shinfo_cache, kvm, true);
}

void kvm_xen_destroy_vm(struct kvm *kvm)
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 9da947eb08ed..7b2dbbd6b104 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -1474,7 +1474,13 @@ int kvm_vcpu_write_guest(struct kvm_vcpu *vcpu, gpa_t gpa, const void *data,
* immutable attributes. Note, the cache must be zero-allocated (or zeroed by
* the caller before init).
*/
-void kvm_gpc_init(struct gfn_to_pfn_cache *gpc, struct kvm *kvm);
+void __kvm_gpc_init(struct gfn_to_pfn_cache *gpc, struct kvm *kvm,
+ bool never_dirty);
+
+static inline void kvm_gpc_init(struct gfn_to_pfn_cache *gpc, struct kvm *kvm)
+{
+ __kvm_gpc_init(gpc, kvm, false);
+}

/**
* kvm_gpc_activate - prepare a cached kernel mapping and HPA for a given guest
@@ -1983,7 +1989,7 @@ static inline void kvm_gpc_mark_dirty_in_slot(struct gfn_to_pfn_cache *gpc)
RCU_LOCKDEP_WARN(!srcu_read_lock_held(&gpc->kvm->gpc_srcu),
"kvm_gpc_mark_dirty_in_slot() without kvm->gpc_srcu read lock");

- if (!gpc->memslot)
+ if (!gpc->memslot || gpc->never_dirty)
return;

mark_page_dirty_in_slot(gpc->kvm, gpc->memslot, gpa_to_gfn(gpc->gpa));
diff --git a/include/linux/kvm_types.h b/include/linux/kvm_types.h
index 31f3eb0dd2aa..22393f97266a 100644
--- a/include/linux/kvm_types.h
+++ b/include/linux/kvm_types.h
@@ -109,6 +109,7 @@ struct gfn_to_pfn_cache {
* instead.
*/
atomic_t state;
+ bool never_dirty;
};

/* The cache is valid: pfn and khva may be used by readers. */
diff --git a/virt/kvm/pfncache.c b/virt/kvm/pfncache.c
index a6a7cfd13246..97958af667fb 100644
--- a/virt/kvm/pfncache.c
+++ b/virt/kvm/pfncache.c
@@ -597,7 +597,8 @@ int kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, unsigned long len)
return __kvm_gpc_refresh(gpc, gpc->gpa, uhva);
}

-void kvm_gpc_init(struct gfn_to_pfn_cache *gpc, struct kvm *kvm)
+void __kvm_gpc_init(struct gfn_to_pfn_cache *gpc, struct kvm *kvm,
+ bool never_dirty)
{
mutex_init(&gpc->refresh_lock);

@@ -607,6 +608,7 @@ void kvm_gpc_init(struct gfn_to_pfn_cache *gpc, struct kvm *kvm)
gpc->uhva = KVM_HVA_ERR_BAD;
gpc->active = false;
atomic_set(&gpc->state, 0);
+ gpc->never_dirty = never_dirty;
}

static int __kvm_gpc_activate(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned long uhva,
--
2.55.0