Re: [PATCH v13 34/85] KVM: Get writable mapping for __kvm_vcpu_map() only when necessary

Yan Zhao
2024-10-22T01:53:58Z

On Mon, Oct 21, 2024 at 11:13:08AM -0700, Sean Christopherson wrote:
> On Mon, Oct 21, 2024, Yan Zhao wrote:
> > On Thu, Oct 10, 2024 at 11:23:36AM -0700, Sean Christopherson wrote:
> > > When creating a memory map for read, don't request a writable pfn from the
> > > primary MMU. While creating read-only mappings can be theoretically slower,
> > > as they don't play nice with fast GUP due to the need to break CoW before
> > > mapping the underlying PFN, practically speaking, creating a mapping isn't
> > > a super hot path, and getting a writable mapping for reading is weird and
> > > confusing.
> > >
> > > Tested-by: Alex Bennée <alex.bennee@linaro.org>
> > > Signed-off-by: Sean Christopherson <seanjc@google.com>
> > > ---
> > > virt/kvm/kvm_main.c | 2 +-
> > > 1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > > index 080740f65061..b845e9252633 100644
> > > --- a/virt/kvm/kvm_main.c
> > > +++ b/virt/kvm/kvm_main.c
> > > @@ -3122,7 +3122,7 @@ int __kvm_vcpu_map(struct kvm_vcpu *vcpu, gfn_t gfn, struct kvm_host_map *map,
> > > struct kvm_follow_pfn kfp = {
> > > .slot = gfn_to_memslot(vcpu->kvm, gfn),
> > > .gfn = gfn,
> > > - .flags = FOLL_WRITE,
> > > + .flags = writable ? FOLL_WRITE : 0,
> > > .refcounted_page = &map->pinned_page,
> > > .pin = true,
> > > };
> > When writable is false, could we set ".pin = false," ?
>
> Hmm, maybe? I can't imagine anything would actually break, but unless FOLL_PIN
> implies writing, my preference would still be to pin the page so that KVM always
> pins when accessing the actual data of a page.
Ok. So setting .pin = true here is because of KVM direct access, which does not
check mmu notifier's invalidation callback.