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

Tom Lendacky
2024-01-12T20:28:20Z

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...
>