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

[Dave Hansen]

On 7/15/21 11:56 AM, Sean Christopherson wrote:
>>>> +       /* 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 was envisioning that two different CPUs could try to smash two
*different* 4k physical pages, but collide since they share
a 2M page.

But, in patch 33, this is called via:

> +		write_lock(&kvm->mmu_lock);
> +
> +		switch (op) {
> +		case SNP_PAGE_STATE_SHARED:
> +			rc = snp_make_page_shared(vcpu, gpa, pfn, level);
...

Which should make collisions impossible.  Did I miss another call-site?