Re: [PATCH v2 4/8] KVM: pfncache: simplify locking and make more self-contained

[Sean Christopherson <seanjc@google.com>]

On Tue, Feb 27, 2024, David Woodhouse wrote:
>  static int __kvm_gpc_activate(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned long uhva,
>  			      unsigned long len)
>  {
>  	struct kvm *kvm = gpc->kvm;
> +	int ret;
> +
> +	mutex_lock(&gpc->refresh_lock);
>  
>  	if (!gpc->active) {
>  		if (KVM_BUG_ON(gpc->valid, kvm))

Heh, it's _just_ out of sight in this diff, but this has an error path that misses
the unlock.  The full context is:

	if (!gpc->active) {
		if (KVM_BUG_ON(gpc->valid, kvm))
			return -EIO;

At the risk of doing too much when applying, I think this is the perfect patch to
start introducing use of guard(mutex) in common KVM.  As a bonus, it's a noticeably
smaller diff.  Testing this now...

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>
Link: https://lore.kernel.org/r/20240227115648.3104-5-dwmw2@infradead.org
[sean: use guard(mutex) to fix a missed unlock]
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 virt/kvm/pfncache.c | 21 +++++++++++----------
 1 file changed, 11 insertions(+), 10 deletions(-)

diff --git a/virt/kvm/pfncache.c b/virt/kvm/pfncache.c
index 9ac8c9da4eda..4e07112a24c2 100644
--- a/virt/kvm/pfncache.c
+++ b/virt/kvm/pfncache.c
@@ -256,12 +256,7 @@ static int __kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned l
 	if (page_offset + len > PAGE_SIZE)
 		return -EINVAL;
 
-	/*
-	 * If another task is refreshing the cache, wait for it to complete.
-	 * There is no guarantee that concurrent refreshes will see the same
-	 * gpa, memslots generation, etc..., so they must be fully serialized.
-	 */
-	mutex_lock(&gpc->refresh_lock);
+	lockdep_assert_held(&gpc->refresh_lock);
 
 	write_lock_irq(&gpc->lock);
 
@@ -347,8 +342,6 @@ static int __kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned l
 out_unlock:
 	write_unlock_irq(&gpc->lock);
 
-	mutex_unlock(&gpc->refresh_lock);
-
 	if (unmap_old)
 		gpc_unmap(old_pfn, old_khva);
 
@@ -357,13 +350,16 @@ static int __kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned l
 
 int kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, unsigned long len)
 {
+	unsigned long uhva;
+
+	guard(mutex)(&gpc->refresh_lock);
+
 	/*
 	 * If the GPA is valid then ignore the HVA, as a cache can be GPA-based
 	 * or HVA-based, not both.  For GPA-based caches, the HVA will be
 	 * recomputed during refresh if necessary.
 	 */
-	unsigned long uhva = kvm_is_error_gpa(gpc->gpa) ? gpc->uhva :
-							  KVM_HVA_ERR_BAD;
+	uhva = kvm_is_error_gpa(gpc->gpa) ? gpc->uhva : KVM_HVA_ERR_BAD;
 
 	return __kvm_gpc_refresh(gpc, gpc->gpa, uhva, len);
 }
@@ -377,6 +373,7 @@ void kvm_gpc_init(struct gfn_to_pfn_cache *gpc, struct kvm *kvm)
 	gpc->pfn = KVM_PFN_ERR_FAULT;
 	gpc->gpa = INVALID_GPA;
 	gpc->uhva = KVM_HVA_ERR_BAD;
+	gpc->active = gpc->valid = false;
 }
 
 static int __kvm_gpc_activate(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned long uhva,
@@ -384,6 +381,8 @@ static int __kvm_gpc_activate(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned
 {
 	struct kvm *kvm = gpc->kvm;
 
+	guard(mutex)(&gpc->refresh_lock);
+
 	if (!gpc->active) {
 		if (KVM_BUG_ON(gpc->valid, kvm))
 			return -EIO;
@@ -420,6 +419,8 @@ void kvm_gpc_deactivate(struct gfn_to_pfn_cache *gpc)
 	kvm_pfn_t old_pfn;
 	void *old_khva;
 
+	guard(mutex)(&gpc->refresh_lock);
+
 	if (gpc->active) {
 		/*
 		 * Deactivate the cache before removing it from the list, KVM

base-commit: 6d6f106db109251010423d8728d76a0260db5814
--