Re: [PATCH] KVM: SEV: Add cond_resched() to loop in sev_clflush_pages()

[Peter Gonda]

On Wed, Apr 6, 2022 at 12:26 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> On Wed, Apr 06, 2022, Mingwei Zhang wrote:
> > Hi Sean,
> >
> > > > > diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> > > > > index 75fa6dd268f0..c2fe89ecdb2d 100644
> > > > > --- a/arch/x86/kvm/svm/sev.c
> > > > > +++ b/arch/x86/kvm/svm/sev.c
> > > > > @@ -465,6 +465,7 @@ static void sev_clflush_pages(struct page *pages[], unsigned long npages)
> > > > >                 page_virtual = kmap_atomic(pages[i]);
> > > > >                 clflush_cache_range(page_virtual, PAGE_SIZE);
> > > > >                 kunmap_atomic(page_virtual);
> > > > > +               cond_resched();
> > > >
> > > > If you add cond_resched() here, the frequency (once per 4K) might be
> > > > too high. You may want to do it once per X pages, where X could be
> > > > something like 1G/4K?
> > >
> > > No, every iteration is perfectly ok.  The "cond"itional part means that this will
> > > reschedule if and only if it actually needs to be rescheduled, e.g. if the task's
> > > timeslice as expired.  The check for a needed reschedule is cheap, using
> > > cond_resched() in tight-ish loops is ok and intended, e.g. KVM does a reched
> > > check prior to enterring the guest.
> >
> > Double check on the code again. I think the point is not about flag
> > checking. Obviously branch prediction could really help. The point I
> > think is the 'call' to cond_resched(). Depending on the kernel
> > configuration, cond_resched() may not always be inlined, at least this
> > is my understanding so far? So if that is true, then it still might
> > not always be the best to call cond_resched() that often.
>
> Eh, compared to the cost of 64 back-to-back CLFLUSHOPTs, the cost of __cond_resched()
> is peanuts.  Even accounting for the rcu_all_qs() work, it's still dwarfed by the
> cost of flushing data from the cache.  E.g. based on Agner Fog's wonderful uop
> latencies[*], the actual flush time for a single page is going to be upwards of
> 10k cycles, whereas __cond_resched() is going to well under 100 cycles in the happy
> case of no work.  Even if those throughput numbers are off by an order of magnitude,
> e.g. CLFLUSHOPT can complete in 15 cycles, that's still ~1k cycles.
>
> Peter, don't we also theoretically need cond_resched() in the loops in
> sev_launch_update_data()?  AFAICT, there's no articifical restriction on the size
> of the payload, i.e. the kernel is effectively relying on userspace to not update
> large swaths of memory.

Yea we probably do want to cond_resched() in the for loop inside of
sev_launch_update_data(). Ithink in  sev_dbg_crypt() userspace could
request a large number of pages to be decrypted/encrypted for
debugging but se have a call to sev_pin_memory() in the loop so that
will have a cond_resded() inside of __get_users_pages(). Or should we
have a cond_resded() inside of the loop in sev_dbg_crypt() too?

>
> [*] https://www.agner.org/optimize/instruction_tables.pdf