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

[Sean Christopherson]

On Thu, Jul 15, 2021, Brijesh Singh wrote:
> 
> On 7/14/21 5:25 PM, Sean Christopherson wrote:
> > > @@ -2375,6 +2375,12 @@ int rmpupdate(struct page *page, struct rmpupdate *val)
> > >   	if (!cpu_feature_enabled(X86_FEATURE_SEV_SNP))
> > >   		return -ENXIO;
> > > +	ret = set_memory_4k((unsigned long)page_to_virt(page), 1);
> > 
> > IIUC, this shatters the direct map for page that's assigned to an SNP guest, and
> > the large pages are never recovered?
> > 
> > I believe a better approach would be to do something similar to memfd_secret[*],
> > which encountered a similar problem with the direct map.  Instead of forcing the
> > direct map to be forever 4k, unmap the direct map when making a page guest private,
> > and restore the direct map when it's made shared (or freed).
> > 
> > I thought memfd_secret had also solved the problem of restoring large pages in
> > the direct map, but at a glance I can't tell if that's actually implemented
> > anywhere.  But, even if it's not currently implemented, I think it makes sense
> > to mimic the memfd_secret approach so that both features can benefit if large
> > page preservation/restoration is ever added.
> > 
> 
> thanks for the memfd_secrets pointer. At the lowest level it shares the
> same logic to split the physmap. We both end up calling to
> change_page_attrs_set_clr() which split the page and updates the page
> table attributes.
> 
> Given this, I believe in future if the change_page_attrs_set_clr() is
> enhanced to track the splitting of the pages and restore it later then it
> should work transparently.

But something actually needs to initiate the restore.  If the RMPUDATE path just
force 4k pages then there will never be a restore.  And zapping the direct map
for private pages is a good thing, e.g. prevents the kernel from reading garbage,
which IIUC isn't enforced by the RMP?