Re: [PATCH] x86: refine debugging of SMEP/SMAP fix

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 17/05/16 11:42, Jan Beulich wrote:
>>>> On 17.05.16 at 12:28, <andrew.cooper3@citrix.com> wrote:
>> On 17/05/16 10:54, Jan Beulich wrote:
>>> Instead of just latching cr4_pv32_mask into %rdx, correct the found
>>> wrong value in %cr4 (to avoid triggering another BUG). The value left
>>> in %rdx should be sufficient for deducing cr4_pv32_mask from the
>>> register dump.
>> Alternatively, you can reuse %rax (as its value is useless by this
>> point) and leave %rdx as exactly cr4_pv32_mask.  This avoids needing a
>> subsequent step to reverse engineer cr4_pv32_mask.
> I don't view the value in %rax as useless - that's the set of bits
> we have found set, which didn't match our expectation. Hence I
> specifically don't want to re-use that register.

Ok.  We don't need to preserve any registers from the caller, so using a
different one such as %rbx or %rcx would be fine.

The issue is that it is important to see precisely what cr4_pv32_mask is.

~Andrew

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