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,
 	};
-- 
2.47.0.rc1.288.g06298d1525-goog

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," ?

Also not sure if ".map_writable = NULL" is missing.

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.

> Also not sure if ".map_writable = NULL" is missing.

Doh, my previous response was slightly wrong, it's implicitly initialized to NULL,
not false.  I forgot map_writable is a pointer to a bool.

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.