Re: [PATCH v2] x86emul: de-duplicate scatters to the same linear address

["Roger Pau Monné" <roger.pau@citrix.com>]

On Mon, Oct 18, 2021 at 02:17:39PM +0200, Jan Beulich wrote:
> On 18.10.2021 13:19, Roger Pau Monné wrote:
> > On Thu, May 20, 2021 at 03:34:28PM +0200, Jan Beulich wrote:
> >> The SDM specifically allows for earlier writes to fully overlapping
> >> ranges to be dropped. If a guest did so, hvmemul_phys_mmio_access()
> >> would crash it if varying data was written to the same address. Detect
> >> overlaps early, as doing so in hvmemul_{linear,phys}_mmio_access() would
> >> be quite a bit more difficult. To maintain proper faulting behavior,
> >> instead of dropping earlier write instances of fully overlapping slots
> >> altogether, write the data of the final of these slots multiple times.
> > 
> > Is it possible for a later (non duplicated slot) to cause a fault
> > ending the instruction without reaching that final slot that contains
> > the written data?
> 
> Yes, but that's not a problem: Only faults are required to be ordered,
> and when a fault occurs guarantees are made only towards lower indices
> (read: all lower index writes would have completed, while nothing can
> be said about higher indices). All non-faulting writes can go out in
> any order (unless there are [partial] overlaps, but afaict that case
> still gets dealt with within spec by the proposed new logic).

Oh, OK, so it's fine for a later write to be 'completed' even if one
of the previous ones faulted. In that case:

Acked-by: Roger Pau Monné <roger.pau@citrix.com>

Thanks, Roger.