Re: [Xen-devel] [PATCH v2 11/16] x86/PV: split out dealing with DRn from privileged instruction handling

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 29.09.16 at 22:13, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 28/09/16 09:15, Jan Beulich wrote:
>> This is in preparation for using the generic emulator here.
>>
>> Some care is needed temporarily to not unduly alter guest register
>> state: The local variable "res" can only go away once this code got
>> fully switched over to using x86_emulate().
>>
>> Also switch to IS_ERR_VALUE() instead of (incorrectly) open coding it.
> 
> It isn't actually an ERR_PTR().  That bit of code pre-dates the
> introduction of ERR_PTR() by some margin.
> 
> The return code of do_get_debugreg() is broken and needs fixing, along
> with the ABI of the debugreg hypercalls.
> 
> This change does cause an ABI change for PV guests, as they now can't
> read a debug register whose value is in the top 4k of linear address
> space, (ather than the top 8th of a page previously), but given that the
> ABI is already known broken, I am not sure I care too much.

Well, the way ERR_PTR() works we imply the valid errno range to
be -1...-4095. I.e. assuming a smaller range here is a latent bug
(becoming an actual one if any such value would get bubbled up
through that path). So I prefer to keep the patch the way it is,
despite its ABI effect - the implication of PV guests not being able
to fully use breakpoints on the last 8th of a page clearly has the
implication that no breakpoint should be put in the last page
anyway (and quite likely no PV guest OS has that page mapped in
the first place).

Jan


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