KVM: Drop KVM's API to allow temporarily unmapping gfn=>pfn cache

Drop kvm_gpc_unmap() as it has no users and unclear requirements.  The
API was added as part of the original gfn_to_pfn_cache support, but its
sole usage[*] was never merged.  Fold the guts of kvm_gpc_unmap() into
the deactivate path and drop the API.  Omit acquiring refresh_lock as
as concurrent calls to kvm_gpc_deactivate() are not allowed (this is
not enforced, e.g. via lockdep. due to it being called during vCPU
destruction).

If/when temporary unmapping makes a comeback, the desirable behavior is
likely to restrict temporary unmapping to vCPU-exclusive mappings and
require the vcpu->mutex be held to serialize unmap.  Use of the
refresh_lock to protect unmapping was somewhat specuatively added by
commit 93984f19e7bc ("KVM: Fully serialize gfn=>pfn cache refresh via
mutex") to guard against concurrent unmaps, but the primary use case of
the temporary unmap, nested virtualization[*], doesn't actually need or
want concurrent unmaps.

[*] https://lore.kernel.org/all/20211210163625.2886-7-dwmw2@infradead.org

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 3ce4650..eac7696 100644 (file)
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -1334,18 +1334,6 @@ bool kvm_gpc_check(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned long len);
 int kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned long len);
 
 /**
- * kvm_gpc_unmap - temporarily unmap a gfn_to_pfn_cache.
- *
- * @kvm:          pointer to kvm instance.
- * @gpc:          struct gfn_to_pfn_cache object.
- *
- * This unmaps the referenced page. The cache is left in the invalid state
- * but at least the mapping from GPA to userspace HVA will remain cached
- * and can be reused on a subsequent refresh.
- */
-void kvm_gpc_unmap(struct kvm *kvm, struct gfn_to_pfn_cache *gpc);
-
-/**
  * kvm_gpc_deactivate - deactivate and unlink a gfn_to_pfn_cache.
  *
  * @gpc:          struct gfn_to_pfn_cache object.

diff --git a/virt/kvm/pfncache.c b/virt/kvm/pfncache.c
index 015c5d1..5b25127 100644 (file)
--- a/virt/kvm/pfncache.c
+++ b/virt/kvm/pfncache.c
@@ -333,33 +333,6 @@ out_unlock:
 }
 EXPORT_SYMBOL_GPL(kvm_gpc_refresh);
 
-void kvm_gpc_unmap(struct kvm *kvm, struct gfn_to_pfn_cache *gpc)
-{
-       void *old_khva;
-       kvm_pfn_t old_pfn;
-
-       mutex_lock(&gpc->refresh_lock);
-       write_lock_irq(&gpc->lock);
-
-       gpc->valid = false;
-
-       old_khva = gpc->khva - offset_in_page(gpc->khva);
-       old_pfn = gpc->pfn;
-
-       /*
-        * We can leave the GPA → uHVA map cache intact but the PFN
-        * lookup will need to be redone even for the same page.
-        */
-       gpc->khva = NULL;
-       gpc->pfn = KVM_PFN_ERR_FAULT;
-
-       write_unlock_irq(&gpc->lock);
-       mutex_unlock(&gpc->refresh_lock);
-
-       gpc_unmap_khva(old_pfn, old_khva);
-}
-EXPORT_SYMBOL_GPL(kvm_gpc_unmap);
-
 void kvm_gpc_init(struct gfn_to_pfn_cache *gpc, struct kvm *kvm,
                  struct kvm_vcpu *vcpu, enum pfn_cache_usage usage)
 {
@@ -405,6 +378,8 @@ EXPORT_SYMBOL_GPL(kvm_gpc_activate);
 void kvm_gpc_deactivate(struct gfn_to_pfn_cache *gpc)
 {
        struct kvm *kvm = gpc->kvm;
+       kvm_pfn_t old_pfn;
+       void *old_khva;
 
        if (gpc->active) {
                /*
@@ -414,13 +389,26 @@ void kvm_gpc_deactivate(struct gfn_to_pfn_cache *gpc)
                 */
                write_lock_irq(&gpc->lock);
                gpc->active = false;
+               gpc->valid = false;
+
+               /*
+                * Leave the GPA => uHVA cache intact, it's protected by the
+                * memslot generation.  The PFN lookup needs to be redone every
+                * time as mmu_notifier protection is lost when the cache is
+                * removed from the VM's gpc_list.
+                */
+               old_khva = gpc->khva - offset_in_page(gpc->khva);
+               gpc->khva = NULL;
+
+               old_pfn = gpc->pfn;
+               gpc->pfn = KVM_PFN_ERR_FAULT;
                write_unlock_irq(&gpc->lock);
 
                spin_lock(&kvm->gpc_lock);
                list_del(&gpc->list);
                spin_unlock(&kvm->gpc_lock);
 
-               kvm_gpc_unmap(kvm, gpc);
+               gpc_unmap_khva(old_pfn, old_khva);
        }
 }
 EXPORT_SYMBOL_GPL(kvm_gpc_deactivate);