Re: [Xen-devel] [PATCH v4 17/17] x86/hvm: track large memory mapped accesses by buffer offset

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 24.06.15 at 13:24, <paul.durrant@xxxxxxxxxx> wrote:
> @@ -621,14 +574,41 @@ static int hvmemul_phys_mmio_access(
>  
>      for ( ;; )
>      {
> -        rc = hvmemul_do_mmio_buffer(gpa, &one_rep, chunk, dir, 0,
> -                                    *buffer);
> -        if ( rc != X86EMUL_OKAY )
> -            break;
> +        /* Have we already done this chunk? */
> +        if ( (*off + chunk) <= vio->mmio_cache[dir].size )

I can see why you would like to get rid of the address check, but
I'm afraid you can't: You have to avoid getting mixed up multiple
same kind (reads or writes) memory accesses that a single
instruction can do. While generally I would assume that
secondary accesses (like the I/O bitmap read associated with an
OUTS) wouldn't go to MMIO, CMPS with both operands being
in MMIO would break even if neither crosses a page boundary
(not to think of when the emulator starts supporting the
scatter/gather instructions, albeit supporting them will require
further changes, or we could choose to do them one element at
a time).

> +        {
> +            ASSERT(*off + chunk <= vio->mmio_cache[dir].size);

I don't see any difference to the if() expression just above.

> +            if ( dir == IOREQ_READ )
> +                memcpy(&buffer[*off],
> +                       &vio->mmio_cache[IOREQ_READ].buffer[*off],
> +                       chunk);
> +            else
> +            {
> +                if ( memcmp(&buffer[*off],

"else if" please.

> +                            &vio->mmio_cache[IOREQ_WRITE].buffer[*off],
> +                            chunk) != 0 )
> +                    domain_crash(curr->domain);
> +            }
> +        }
> +        else
> +        {
> +            ASSERT(*off == vio->mmio_cache[dir].size);
> +
> +            rc = hvmemul_do_mmio_buffer(gpa, &one_rep, chunk, dir, 0,
> +                                        &buffer[*off]);
> +            if ( rc != X86EMUL_OKAY )
> +                break;
> +
> +            /* Note that we have now done this chunk */

Missing stop.

Jan


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