Re: [Xen-devel] [PATCH v6 16/16] x86/hvm: track large memory mapped accesses by buffer offset

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: 09 July 2015 11:33
> To: Paul Durrant
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; Keir (Xen.org)
> Subject: Re: [PATCH v6 16/16] x86/hvm: track large memory mapped
> accesses by buffer offset
> 
> >>> On 03.07.15 at 18:25, <paul.durrant@xxxxxxxxxx> wrote:
> > @@ -635,13 +605,41 @@ static int hvmemul_phys_mmio_access(
> >      return rc;
> >  }
> >
> > +static struct hvm_mmio_cache *hvmemul_find_mmio_cache(
> > +    struct hvm_vcpu_io *vio, unsigned long gla, uint8_t dir)
> > +{
> > +    unsigned int i;
> > +    struct hvm_mmio_cache *cache;
> > +
> > +    for ( i = 0; i < vio->mmio_cache_count; i ++ )
> > +    {
> > +        cache = &vio->mmio_cache[i];
> > +
> > +        if ( gla == cache->gla &&
> > +             dir == cache->dir )
> > +            return cache;
> > +    }
> > +
> > +    i = vio->mmio_cache_count++;
> > +    BUG_ON(i == ARRAY_SIZE(vio->mmio_cache));
> > +
> > +    cache = &vio->mmio_cache[i];
> > +    memset(cache, 0, sizeof (*cache));
> > +
> > +    cache->gla = gla;
> > +    cache->dir = dir;
> > +
> > +    return cache;
> > +}
> 
> There's still a weakness here (but that also applies if you used physical
> addresses): Multiple reads to the same address may not return the
> same result. I don't think this needs to be addressed here, but adding
> a comment clarifying that this case isn't handled correctly would help
> future readers easier understand the state of affairs.
> 

Yes, the way that multicycle I/O is handled cannot cope with the same 
instruction reading/writing the same place twice. I will add a comment noting 
that as you suggest.

> As to the BUG_ON() - I think this would better be a domain_crash(),
> just in case we overlooked an exotic instruction accessing more than
> 3 memory locations.
> 

Sure.

  Paul

> Jan


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