Re: [PATCH] x86/mm: Don't try to change poison pages to uncacheable in a guest

[Sean Christopherson]

On Mon, May 18, 2020 at 11:26:29AM -0700, Luck, Tony wrote:
> Maybe it isn't pretty. But I don't see another practical solution.
> 
> The VMM is doing exactly the right thing here. It should not trust
> that the guest will behave and not touch the poison location again.
> If/when the guest does touch the poison, the right action is
> for the VMM to fake a new machine check to the guest.
> 
> Theoretlcally the VMM could decode the instruction that the guest
> was trying to use on the poison page and decide "oh, this is that
> weird case in Linux where it's just trying to CLFLUSH the page. I'll
> just step the return IP past the CLFLUSH and let the guest continue".

That's actually doable in KVM, e.g. a hack could be done in <10 lines of
code.  A proper fix that integrates with KVM's emulator would be
substantially more code and effort though.

> But that doesn't sound at all reasonable to me (especially as the
> next step is to realize that Linux is going to repeat that for every
> cache line in the page, so you also want to VMM to fudge the register
> contents to skip to the end of the loop and avoid another 63 VMEXITs).

Eh, 63 VM-Exits is peanuts in the grand scheme.  Even with the host-side
gup() that's probably less than 50us.

> N.B. Linux wants to switch the page to uncacheable so that in the
> persistant memory case the filesytem code can continue to access
> the other "blocks" in the page, rather than lose all of them. That's
> futile in the case where the VMM took the whole 4K away. Maybe Dan
> needs to think about the guest case too.

This is where I'm unclear as to the guest behavior.  Is it doing *just*
CLFLUSH, or is it doing CLFLUSH followed by other accesses to the poisoned
page?  If it's the former, then it's probably worth at least exploring a
KVM fix.  If it's the latter, then yeah, emulating CLFLUSH for a poisoned
#MC is pointless.  I assume it's the latter since the goal is to recover
data?

Oh, and FWIW, the guest won't actually get UC for that page.