Re: [Xen-devel] [PATCH v7 3/5] x86/ioreq server: Handle read-modify-write cases for p2m_ioreq_server pages.

[Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>]

On 3/13/2017 7:22 PM, Jan Beulich wrote:
> > > > On 11.03.17 at 09:42, <yu.c.zhang@xxxxxxxxxxxxxxx> wrote:
> > On 3/10/2017 11:33 PM, Jan Beulich wrote:
> > > > > > On 08.03.17 at 16:33, <yu.c.zhang@xxxxxxxxxxxxxxx> wrote:
> > > > @@ -197,6 +217,10 @@ static int hvmemul_do_io(
> > > >             *   - If the IOREQ_MEM_ACCESS_WRITE flag is not set, treat it
> > > >             *   like a normal PIO or MMIO that doesn't have an ioreq
> > > >             *   server (i.e., by ignoring it).
> > > > +         *
> > > > +         *   - If the accesss is a read, this could be part of a
> > > > +         *   read-modify-write instruction, emulate the read so that we
> > > > +         *   have it.
> > > "it" being what here? Grammatically the insn, but we don't care
> > > about "having" the insn.
> > Here "have it" means " to have the value on this address copied in".
> > Sorry for the inaccurate comment. How about just "emulate the read first"?
> Sounds okay.
>
> > > > @@ -226,6 +250,17 @@ static int hvmemul_do_io(
> > > >                    }
> > > >    
> > > >                    /*
> > > > +                 * This is part of a read-modify-write instruction.
> > > "is" or "may be"?
> > I believe should be "is".
> > It's the only scenario I can imagine when an read operation(only when
> > this operation is
> > also a write one) traps. Otherwise there shall be no VM exit.
> Even with a racing update to the type?
Yes.
With patch 1/5, there shall be no racing update to the p2m type during 
this process.
Besides, I believe even without patch 1/5, and we allow the racing p2m 
change, it will
only happen on a p2m_ioreq_server page changed to p2m_ram_rw(in such 
case it is shall
only be a write or a read-modify-write op that cause such vm exit). The 
race will not
happen for a p2m_ram_rw page changed to p2m_ioreq_server, because there 
shall be no
vm exit at all.

Thanks
Yu

> Jan
>
>
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