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Re: [PATCH 1/3] tools/libacpi: Use 64-byte alignment for FACS
- To: Kevin Stefanov <kevin.stefanov@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Thu, 9 Sep 2021 18:03:39 +0100
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- Cc: Jan Beulich <jbeulich@xxxxxxxx>
- Delivery-date: Thu, 09 Sep 2021 17:04:04 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 09/09/2021 17:34, Kevin Stefanov wrote:
> The spec requires 64-byte alignment, not 16.
>
> Signed-off-by: Kevin Stefanov <kevin.stefanov@xxxxxxxxxx>
> ---
> CC: Jan Beulich <jbeulich@xxxxxxxx>
> CC: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>
> Note: This does not fix the FACS alignment inside guests yet. See next
> patch.
The history here is complex.
c/s 938cee9d41d3 in 2006 deleted a comment saying
// FACS: should be 64-bit alignment
// so it is put at the start of buffer
// as the buffer is 64 bit alignment
Clearly it means byte rather than bit, but the change also introduced
logic to the effect of buf += ROUNDUP(sizeof(facs), 16).
This (incorrect) alignment survived several morphs of the logic and was
moved from hvmloader into libacpi by c/s 73b72736e6ca.
The current code is clearly wrong, but happens to work correctly in
hvmloader because FACS is the first table written and it starts on a
page boundary. The logic does not work correctly when libxl passes a
buffer which doesn't start on a page boundary.
As a consequence, I'm not sure what to suggest as a Fixes tag here,
except "the logic has been wrong for 15 years - patch everything".
~Andrew
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