Re: [PATCH Part2 RFC v4 06/40] x86/sev: Add helper functions for RMPUPDATE and PSMASH instruction

[Sean Christopherson]

On Mon, Jul 12, 2021, Dave Hansen wrote:
> On 7/12/21 11:44 AM, Peter Gonda wrote:
> >> +int psmash(struct page *page)
> >> +{
> >> +       unsigned long spa = page_to_pfn(page) << PAGE_SHIFT;
> >> +       int ret;
> >> +
> >> +       if (!cpu_feature_enabled(X86_FEATURE_SEV_SNP))
> >> +               return -ENXIO;
> >> +
> >> +       /* Retry if another processor is modifying the RMP entry. */
> >> +       do {
> >> +               /* Binutils version 2.36 supports the PSMASH mnemonic. */
> >> +               asm volatile(".byte 0xF3, 0x0F, 0x01, 0xFF"
> >> +                             : "=a"(ret)
> >> +                             : "a"(spa)
> >> +                             : "memory", "cc");
> >> +       } while (ret == FAIL_INUSE);
> > Should there be some retry limit here for safety? Or do we know that
> > we'll never be stuck in this loop? Ditto for the loop in rmpupdate.
> 
> It's probably fine to just leave this.  While you could *theoretically*
> lose this race forever, it's unlikely to happen in practice.  If it
> does, you'll get an easy-to-understand softlockup backtrace which should
> point here pretty quickly.

But should failure here even be tolerated?  The TDX cases spin on flows that are
_not_ due to (direct) contenion, e.g. a pending interrupt while flushing the
cache or lack of randomness when generating a key.  In this case, there are two
CPUs racing to modify the RMP entry, which implies that the final state of the
RMP entry is not deterministic.

> I think TDX has a few of these as well.  Most of the "SEAMCALL"s from
> host to the firmware doing the security enforcement have something like
> an -EBUSY as well.  I believe they just retry forever too.