Re: [Xen-devel] [PATCH v2 4/5] x86/pv: Drop {compat_, }create_bounce_frame() and use the C version instead

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 07.09.18 at 16:17, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 05/03/18 09:27, Jan Beulich wrote:
>>>>> On 27.02.18 at 15:50, <andrew.cooper3@xxxxxxxxxx> wrote:
>>> -compat_create_bounce_frame:
>>> -        ASSERT_INTERRUPTS_ENABLED
>>> -        mov   %fs,%edi
>>> -        ASM_STAC
>>> -        testb $2,UREGS_cs+8(%rsp)
>>> -        jz    1f
>>> -        /* Push new frame at registered guest-OS stack base. */
>>> -        movl  VCPU_kernel_sp(%rbx),%esi
>>> -.Lft1:  mov   VCPU_kernel_ss(%rbx),%fs
>> Note how we did take into consideration the segment base here;
>> pv_create_bounce_frame() doesn't. Hence while the patch here
>> is
>> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
>> I'm afraid I have to withdraw the respective tag for the earlier one
>> (despite realizing that there are other places where we [wrongly]
>> assume stack segments to be flat).
> 
> For the failsafe callback, %ss is set to be flat, and then a bounce
> frame is created at the current kernel_sp.
> 
> Despite the impression the API might give, a 32bit PV kernel cannot use
> a non-flat stack segment.  No PV guest (not even MiniOS) uses a non-flat
> layout, so while this is a change of behaviour, its not going to break
> anything.

Well, at the very least such a change in behavior needs to be called
out very prominently in the description.

Jan



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