Re: [PATCH] x86/flushtlb: remove flush_area check on system state

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, May 18, 2022 at 10:49:22AM +0200, Jan Beulich wrote:
> On 16.05.2022 16:31, Roger Pau Monne wrote:
> > --- a/xen/arch/x86/include/asm/flushtlb.h
> > +++ b/xen/arch/x86/include/asm/flushtlb.h
> > @@ -146,7 +146,8 @@ void flush_area_mask(const cpumask_t *, const void *va, 
> > unsigned int flags);
> >  #define flush_mask(mask, flags) flush_area_mask(mask, NULL, flags)
> >  
> >  /* Flush all CPUs' TLBs/caches */
> > -#define flush_area_all(va, flags) flush_area_mask(&cpu_online_map, va, 
> > flags)
> > +#define flush_area(va, flags) \
> > +    flush_area_mask(&cpu_online_map, (const void *)(va), flags)
> 
> I have to admit that I would prefer if we kept the "_all" name suffix,
> to continue to clearly express the scope of the flush. I'm also not
> really happy to see the cast being added globally now.

But there where no direct callers of flush_area_all(), so the name was
just relevant for it's use in flush_area().  With that now gone I
don't see a need for a flush_area_all(), as flush_area_mask() is more
appropriate.

> > --- a/xen/arch/x86/smp.c
> > +++ b/xen/arch/x86/smp.c
> > @@ -262,7 +262,8 @@ void flush_area_mask(const cpumask_t *mask, const void 
> > *va, unsigned int flags)
> >  {
> >      unsigned int cpu = smp_processor_id();
> >  
> > -    ASSERT(local_irq_is_enabled());
> > +    /* Local flushes can be performed with interrupts disabled. */
> > +    ASSERT(local_irq_is_enabled() || cpumask_equal(mask, cpumask_of(cpu)));
> 
> Further down we use cpumask_subset(mask, cpumask_of(cpu)),
> apparently to also cover the case where mask is empty. I think
> you want to do so here as well.

Hm, yes.  I guess that's cheaper than adding an extra:

if ( cpumask_empty() )
    return;

check at the start of the function.

> >      if ( (flags & ~(FLUSH_VCPU_STATE | FLUSH_ORDER_MASK)) &&
> >           cpumask_test_cpu(cpu, mask) )
> 
> I suppose we want a further precaution here: Despite the
> !cpumask_subset(mask, cpumask_of(cpu)) below I think we want to
> extend what c64bf2d2a625 ("x86: make CPU state flush requests
> explicit") and later changes (isolating uses of FLUSH_VCPU_STATE
> from other FLUSH_*) did and exclude the use of FLUSH_VCPU_STATE
> for the local CPU altogether.

If we really want to exclude the use of FLUSH_VCPU_STATE for the local
CPU, we might wish to add this as a separate ASSERT, so that such
checking doesn't depend on !local_irq_is_enabled():

ASSERT(local_irq_is_enabled() || cpumask_subset(mask, cpumask_of(cpu));
ASSERT(!cpumask_subset(mask, cpumask_of(cpu)) || !(flags & FLUSH_VCPU_STATE));


> That's because if such somehow made
> it into the conditional below here, it would still involve an IPI.

Sorry, I'm confused by this: if the mask is empty there should be no
IPI involved at all?  And we shouldn't even get into the second
conditional on the function.

Thanks, Roger.