Re: [RFC Part1 PATCH 13/13] x86/kernel: add support to validate memory when changing C-bit

Borislav Petkov
2021-04-12T13:05:48Z

On Mon, Apr 12, 2021 at 07:55:01AM -0500, Brijesh Singh wrote:
> The cur_entry is updated by the hypervisor. While building the psc
> buffer the guest sets the cur_entry=0 and the end_entry point to the
> last valid entry. The cur_entry is incremented by the hypervisor after
> it successfully processes one 4K page. As per the spec, the hypervisor
> could get interrupted in middle of the page state change and cur_entry
> allows the guest to resume the page state change from the point where it
> was interrupted.

This is non-obvious and belongs in a comment above it. Otherwise it
looks weird.

> Since we can get interrupted while executing the PSC so just to be safe
> I re-initialized the scratch scratch area with our buffer instead of
> relying on old values.

Ditto.

> As per the spec the caller must check that the cur_entry > end_entry to
> determine whether all the entries are processed. If not then retry the
> state change. The hypervisor will skip the previously processed entries.
> The snp_page_state_vmgexit() is implemented to return only after all the
> entries are changed.

Ditto.

This whole mechanism of what the guest does and what the HV does, needs
to be explained in a bigger comment somewhere around there so that it is
clear what's going on.

Thx.

--
Regards/Gruss,
Boris.