Re: [PATCH v1 11/26] x86/sev: Invalidate pages from the direct map when adding them to the RMP table

[Tom Lendacky]

On 1/12/24 14:07, Borislav Petkov wrote:
> On Fri, Jan 12, 2024 at 12:00:01PM -0800, Dave Hansen wrote:
> > On 12/30/23 08:19, Michael Roth wrote:
> > > If the kernel uses a 2MB directmap mapping to write to an address, and
> > > that 2MB range happens to contain a 4KB page that set to private in the
> > > RMP table, that will also lead to a page-fault exception.

I thought there was also a desire to remove the direct map for any pages 
assigned to a guest as private, not just the case that the comment says. 
So updating the comment would probably the best action.

> > I thought we agreed long ago to just demote the whole direct map to 4k
> > on kernels that might need to act as SEV-SNP hosts.  That should be step
> > one and this can be discussed as an optimization later.

Won't this accomplish that without actually demoting a lot of long-live 
kernel related mappings that would never be demoted? I don't think we need 
to demote the whole mapping to 4K.

Thanks,
Tom

> What would be the disadvantage here? Higher TLB pressure when running
> kernel code I guess...