KVM: Drop extra GUP (via check_user_page_hwpoison()) to detect poisoned page

Remove check_user_page_hwpoison() as it's effectively dead code.  Prior to
commit 234b239bea39 ("kvm: Faults which trigger IO release the mmap_sem"),
hva_to_pfn_slow() wasn't actually a slow path in all cases, i.e. would do
get_user_pages_fast() without ever doing slow GUP with FOLL_HWPOISON.

Now that hva_to_pfn_slow() is a straight shot to get_user_pages_unlocked(),
and unconditionally passes FOLL_HWPOISON, it is impossible for hva_to_pfn()
to get an -errno that needs to be morphed to -EHWPOISON.

There are essentially four cases in KVM:

  - npages == 0, then FOLL_NOWAIT, a.k.a. @async, must be true, and thus
    check_user_page_hwpoison() will not be called
  - npages == 1 || npages == -EHWPOISON, all good
  - npages == -EINTR || npages == -EAGAIN, bail early, all good
  - everything else, including -EFAULT, can go down the vma_lookup() path,
    as npages < 0 means KVM went through hva_to_pfn_slow() which passes
    FOLL_HWPOISON

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Tested-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <20241010182427.1434605-16-seanjc@google.com>

1 files changed, 2 insertions, 15 deletions

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 88db55f9a8b6..f5b7fd653341 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2746,14 +2746,6 @@ unsigned long kvm_vcpu_gfn_to_hva_prot(struct kvm_vcpu *vcpu, gfn_t gfn, bool *w
 	return gfn_to_hva_memslot_prot(slot, gfn, writable);
 }
 
-static inline int check_user_page_hwpoison(unsigned long addr)
-{
-	int rc, flags = FOLL_HWPOISON | FOLL_WRITE;
-
-	rc = get_user_pages(addr, 1, flags, NULL);
-	return rc == -EHWPOISON;
-}
-
 /*
  * The fast path to get the writable pfn which will be stored in @pfn,
  * true indicates success, otherwise false is returned.
@@ -2948,14 +2940,10 @@ kvm_pfn_t hva_to_pfn(unsigned long addr, bool interruptible, bool *async,
 		return pfn;
 	if (npages == -EINTR || npages == -EAGAIN)
 		return KVM_PFN_ERR_SIGPENDING;
+	if (npages == -EHWPOISON)
+		return KVM_PFN_ERR_HWPOISON;
 
 	mmap_read_lock(current->mm);
-	if (npages == -EHWPOISON ||
-	      (!async && check_user_page_hwpoison(addr))) {
-		pfn = KVM_PFN_ERR_HWPOISON;
-		goto exit;
-	}
-
 retry:
 	vma = vma_lookup(current->mm, addr);
 
@@ -2972,7 +2960,6 @@ retry:
 			*async = true;
 		pfn = KVM_PFN_ERR_FAULT;
 	}
-exit:
 	mmap_read_unlock(current->mm);
 	return pfn;
 }