Re: [PATCH v10 21/45] x86/mm: Add support to validate memory when changing C-bit

Kirill A. Shutemov
2022-02-15T12:42:40Z

On Sun, Feb 13, 2022 at 01:15:23PM +0100, Borislav Petkov wrote:
> On Fri, Feb 11, 2022 at 11:27:54AM -0600, Brijesh Singh wrote:
> > > Simply have them always present. They will have !0 values on the
> > > respective guest types and 0 otherwise. This should simplify a lot of
> > > code and another unconditionally present u64 won't be the end of the
> > > world.
> > >
> > > Any other aspect I'm missing?
> >
> > I think that's mostly about it. IIUC, the recommendation is to define a
> > new callback in x86_platform_op. The callback will be invoked
> > unconditionally; The default implementation for this callback is NOP;
> > The TDX and SEV will override with the platform specific implementation.
> > I think we may able to handle everything in one callback hook but having
> > pre and post will be a more desirable. Here is why I am thinking so:
> >
> > * On SNP, the page must be invalidated before clearing the _PAGE_ENC
> > from the page table attribute
> >
> > * On SNP, the page must be validated after setting the _PAGE_ENC in the
> > page table attribute.
>
> Right, we could have a pre- and post- callback, if that would make
> things simpler/clearer.
>
> Also, in thinking further about the encryption mask, we could make it a
> *single*, *global* variable called cc_mask which each guest type sets it
> as it wants to.

I don't think it works. TDX and SME/SEV has opposite polarity of the mask.
SME/SEV has to clear the mask to share the page. TDX has to set it.

Making a single global mask only increases confusion.

--
Kirill A. Shutemov