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Re: [Qemu-arm] [RFC PATCH 1/1] memory: Support unaligned accesses on ali
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-arm] [RFC PATCH 1/1] memory: Support unaligned accesses on aligned-only models |
Date: |
Thu, 13 Jul 2017 14:05:33 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.0 |
On 30/06/2017 05:00, Andrew Jeffery wrote:
> This RFC patch stems from a discussion on a patch for an ADC model[1] where it
> was pointed out that I should be able to use the .impl member of
> MemoryRegionOps to constrain how my read() and write() callbacks where
> invoked.
>
> I tried Phil's suggested approach and found I got reads of size 4, but with an
> address that was not 4-byte aligned.
>
> Looking at the source for access_with_adjusted_size() lead to the comment
>
> /* FIXME: support unaligned access? */
>
> which at least suggests that the implementation isn't complete.
>
> So, this patch is a quick and incomplete attempt at resolving the issue to see
> whether I'm on the right track or way off in the weeds.
>
> I've lightly tested it with the ADC model mentioned above, and it appears to
> do
> the right thing (I changed the values generated by the ADC to distinguish
> between the lower and upper 16 bits).
I think the idea is okay.
> + access_addr[0] = align_down(addr, access_size);
> + access_addr[1] = align_up(addr + size, access_size);
> +
> + for (cur = access_addr[0]; cur < access_addr[1]; cur += access_size)
> {
> + uint64_t mask_bounds[2];
> + mask_bounds[0] = MAX(addr, cur) - cur;
> + mask_bounds[1] =
> + MIN(addr + size, align_up(cur + 1, access_size)) - cur;
> +
> + access_mask = (-1ULL << mask_bounds[0] * 8) &
> + (-1ULL >> (64 - mask_bounds[1] * 8));
Please use MAKE_64BIT_MASK.
> + r |= access(mr, cur, &access_value, access_size,
> + (MAX(addr, cur) - addr), access_mask, attrs);
> +
> + /* XXX: Can't do this hack for writes */
> + access_value >>= mask_bounds[0] * 8;
> + }
Can you subtract access_addr[0] from mask_bounds[0] and mask_bounds[1]
(instead of cur) to remove the need for this right shift?
Thanks,
Paolo
- Re: [Qemu-arm] [RFC PATCH 1/1] memory: Support unaligned accesses on aligned-only models,
Paolo Bonzini <=