Add a kvm_follow_pfn() wrapper, kvm_lookup_pfn(), to allow looking up a
gfn=>pfn mapping without the caller getting a reference to any underlying
page.  The API will be used in flows that want to know if a gfn points at
a valid pfn, but don't actually need to do anything with the pfn.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 include/linux/kvm_host.h |  2 ++
 virt/kvm/kvm_main.c      | 16 ++++++++++++++++
 2 files changed, 18 insertions(+)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 82ca0971c156..5a572cef4adc 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -1212,6 +1212,8 @@ static inline void kvm_release_page_unused(struct page *page)
 void kvm_release_page_clean(struct page *page);
 void kvm_release_page_dirty(struct page *page);
 
+kvm_pfn_t kvm_lookup_pfn(struct kvm *kvm, gfn_t gfn);
+
 kvm_pfn_t gfn_to_pfn(struct kvm *kvm, gfn_t gfn);
 kvm_pfn_t gfn_to_pfn_prot(struct kvm *kvm, gfn_t gfn, bool write_fault,
 		      bool *writable);
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 0b3c0bddaa07..ad84dab8c5dc 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -3118,6 +3118,22 @@ kvm_pfn_t kvm_vcpu_gfn_to_pfn(struct kvm_vcpu *vcpu, gfn_t gfn)
 }
 EXPORT_SYMBOL_GPL(kvm_vcpu_gfn_to_pfn);
 
+kvm_pfn_t kvm_lookup_pfn(struct kvm *kvm, gfn_t gfn)
+{
+	struct page *refcounted_page = NULL;
+	struct kvm_follow_pfn kfp = {
+		.slot = gfn_to_memslot(kvm, gfn),
+		.gfn = gfn,
+		.flags = FOLL_WRITE,
+		.refcounted_page = &refcounted_page,
+	};
+	kvm_pfn_t pfn;
+
+	pfn = kvm_follow_pfn(&kfp);
+	kvm_release_page_unused(refcounted_page);
+	return pfn;
+}
+
 int kvm_prefetch_pages(struct kvm_memory_slot *slot, gfn_t gfn,
 		       struct page **pages, int nr_pages)
 {
-- 
2.46.0.rc1.232.g9752f9e123-goog

On 7/27/24 01:51, Sean Christopherson wrote:
> Add a kvm_follow_pfn() wrapper, kvm_lookup_pfn(), to allow looking up a
> gfn=>pfn mapping without the caller getting a reference to any underlying
> page.  The API will be used in flows that want to know if a gfn points at
> a valid pfn, but don't actually need to do anything with the pfn.

Can you rename the function kvm_gfn_has_pfn(), or 
kvm_gfn_can_be_mapped(), and make it return a bool?

(As an aside, I wonder if reexecute_instruction() could just use 
kvm_is_error_hva(kvm_vcpu_gfn_to_hva(vcpu, gpa_to_gfn(gpa)) instead of 
going all the way to a pfn.  But it's ok to be more restrictive).

Paolo

On Tue, Jul 30, 2024, Paolo Bonzini wrote:
> On 7/27/24 01:51, Sean Christopherson wrote:
> > Add a kvm_follow_pfn() wrapper, kvm_lookup_pfn(), to allow looking up a
> > gfn=>pfn mapping without the caller getting a reference to any underlying
> > page.  The API will be used in flows that want to know if a gfn points at
> > a valid pfn, but don't actually need to do anything with the pfn.
> 
> Can you rename the function kvm_gfn_has_pfn(), or kvm_gfn_can_be_mapped(),
> and make it return a bool?

Heh, sure.  I initially planned on having it return a bool, but I couldn't figure
out a name, mainly because the kernel's pfn_valid() makes things like
kvm_gfn_has_valid_pfn() confusing/misleading :-(

> (As an aside, I wonder if reexecute_instruction() could just use
> kvm_is_error_hva(kvm_vcpu_gfn_to_hva(vcpu, gpa_to_gfn(gpa)) instead of going
> all the way to a pfn.  But it's ok to be more restrictive).

Heh #2, I wondered the same thing.  I think it would work?  Verifying that there's
a usable pfn also protects against retrying an access that hit -EHWPOISON, but I'm
prety sure that would require a rare race, and I don't think it could result in
the guest being put into an infinite loop.

On 7/30/24 22:15, Sean Christopherson wrote:
> On Tue, Jul 30, 2024, Paolo Bonzini wrote:
>> On 7/27/24 01:51, Sean Christopherson wrote:
>>> Add a kvm_follow_pfn() wrapper, kvm_lookup_pfn(), to allow looking up a
>>> gfn=>pfn mapping without the caller getting a reference to any underlying
>>> page.  The API will be used in flows that want to know if a gfn points at
>>> a valid pfn, but don't actually need to do anything with the pfn.
>>
>> Can you rename the function kvm_gfn_has_pfn(), or kvm_gfn_can_be_mapped(),
>> and make it return a bool?
> 
> Heh, sure.  I initially planned on having it return a bool, but I couldn't figure
> out a name, mainly because the kernel's pfn_valid() makes things like
> kvm_gfn_has_valid_pfn() confusing/misleading :-(
> 
>> (As an aside, I wonder if reexecute_instruction() could just use
>> kvm_is_error_hva(kvm_vcpu_gfn_to_hva(vcpu, gpa_to_gfn(gpa)) instead of going
>> all the way to a pfn.  But it's ok to be more restrictive).
> 
> Heh #2, I wondered the same thing.  I think it would work?  Verifying that there's
> a usable pfn also protects against retrying an access that hit -EHWPOISON, but I'm
> prety sure that would require a rare race, and I don't think it could result in
> the guest being put into an infinite loop.

Indeed, and even the check in kvm_alloc_apic_access_page() is totally 
useless.  The page can go away at any time between the call and 
vmx_set_apic_access_page_addr() or, for AMD, the #NPF on 
APIC_DEFAULT_PHYS_BASE.

Yes, it's verifying that the system isn't under extreme memory pressure, 
but in practice a 4K get_user_pages is never going to fail, it's just 
going to cause something else to be swapped.  I'd just get rid of both 
of them, so there's no need for kvm_lookup_pfn().

Paolo