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

[Dave Hansen]

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.

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.