Re: [PATCH Part2 v5 39/45] KVM: SVM: Introduce ops for the post gfn map and unmap

[Sean Christopherson]

On Wed, Oct 13, 2021, Sean Christopherson wrote:
> On Fri, Aug 20, 2021, Brijesh Singh wrote:
> > When SEV-SNP is enabled in the guest VM, the guest memory pages can
> > either be a private or shared. A write from the hypervisor goes through
> > the RMP checks. If hardware sees that hypervisor is attempting to write
> > to a guest private page, then it triggers an RMP violation #PF.
> > 
> > To avoid the RMP violation, add post_{map,unmap}_gfn() ops that can be
> > used to verify that its safe to map a given guest page. Use the SRCU to
> > protect against the page state change for existing mapped pages.
> 
> SRCU isn't protecting anything.  The synchronize_srcu_expedited() in the PSC code
> forces it to wait for existing maps to go away, but it doesn't prevent new maps
> from being created while the actual RMP updates are in-flight.  Most telling is
> that the RMP updates happen _after_ the synchronize_srcu_expedited() call.

Argh, another goof on my part.  Rereading prior feedback, I see that I loosely
suggested SRCU as a possible solution.  That was a bad, bad suggestion.  I think
(hope) I made it offhand without really thinking it through.  SRCU can't work in
this case, because the whole premise of Read-Copy-Update is that there can be
multiple copies of the data.  That simply can't be true for the RMP as hardware
operates on a single table.

In the future, please don't hesitate to push back on and/or question suggestions,
especially those that are made without concrete examples, i.e. are likely off the
cuff.  My goal isn't to set you up for failure :-/