Re: [PATCH Part2 RFC v4 07/40] x86/sev: Split the physmap when adding the page in RMP table

[Brijesh Singh]

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);
	
	...
}

thanks