Re: [RFC PATCH v2 03/10] KVM: Implement kvm_(read|/write)_guest_page for private memory slots

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Hi Sean,

On Fri, 16 Aug 2024 at 20:32, Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> On Thu, Aug 01, 2024, Fuad Tabba wrote:
> > Make __kvm_read_guest_page/__kvm_write_guest_page capable of
> > accessing guest memory if no userspace address is available.
> > Moreover, check that the memory being accessed is shared with the
> > host before attempting the access.
> >
> > KVM at the host might need to access shared memory that is not
> > mapped in the host userspace but is in fact shared with the host,
> > e.g., when accounting for stolen time. This allows the access
> > without relying on the slot's userspace_addr being set.
>
> Why?  As evidenced by the amount of code below, special casing guest_memfd isn't
> trivial, and taking kvm->slots_lock is likely a complete non-starter.  In the
> happy case, uaccess is about as fast as can be, and has no inherent scaling issues.
>
> > This does not circumvent protection, since the access is only
> > attempted if the memory is mappable by the host, which implies
> > shareability.
> >
> > Signed-off-by: Fuad Tabba <tabba@xxxxxxxxxx>
> > ---
> >  virt/kvm/kvm_main.c | 127 ++++++++++++++++++++++++++++++++++++++------
> >  1 file changed, 111 insertions(+), 16 deletions(-)
> >
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index f4b4498d4de6..ec6255c7325e 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -3385,20 +3385,108 @@ int kvm_gmem_clear_mappable(struct kvm *kvm, gfn_t start, gfn_t end)
> >       return kvm_gmem_toggle_mappable(kvm, start, end, false);
> >  }
> >
> > +static int __kvm_read_private_guest_page(struct kvm *kvm,
>
> The changelog says this is for accessing memory that is shared, but this says
> "private".

This is bad naming on my part. Instead, I should call this function
something like, read_guestmem_page (and similar for the write one).
Thanks for pointing this out.

Cheers,
/fuad

> > +                                      struct kvm_memory_slot *slot,
> > +                                      gfn_t gfn, void *data, int offset,
> > +                                      int len)




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