On Thu, Jul 15, 2021, Brijesh Singh wrote:
>
>
> On 7/15/21 1:39 PM, Sean Christopherson wrote:
> > On Thu, Jul 15, 2021, Brijesh Singh wrote:
> > > The memfd_secrets uses the set_direct_map_{invalid,default}_noflush() and it
> > > is designed to remove/add the present bit in the direct map. We can't use
> > > them, because in our case the page may get accessed by the KVM (e.g
> > > kvm_guest_write, kvm_guest_map etc).
> >
> > But KVM should never access a guest private page, i.e. the direct map should
> > always be restored to PRESENT before KVM attempts to access the page.
> >
>
> Yes, KVM should *never* access the guest private pages. So, we could
> potentially enhance the RMPUPDATE() to check for the assigned and act
> accordingly.
>
> Are you thinking something along the line of this:
>
> int rmpupdate(struct page *page, struct rmpupdate *val)
> {
> ...
>
> /*
> * If page is getting assigned in the RMP entry then unmap
> * it from the direct map before its added in the RMP table.
> */
> if (val.assigned)
> set_direct_map_invalid_noflush(page_to_virt(page), 1);
>
> ...
>
> /*
> * If the page is getting unassigned then restore the mapping
> * in the direct map after its removed from the RMP table.
> */
> if (!val.assigned)
> set_direct_map_default_noflush(page_to_virt(page), 1);
>
> ...
> }
Yep.
However, looking at the KVM usage, rmpupdate() appears to be broken. When
handling a page state change, the guest can specify a 2mb page. In that case,
rmpupdate() will be called once for a 2mb page, but this flow assumes a single
4kb page. The current code works because set_memory_4k() will cause the entire
2mb page to be shattered, but it's technically wrong and switching to the above
would cause problems.
