Re: [PATCH v2] x86/pv: Fix assertions in svm_load_segs()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 08/09/2020 16:36, Jan Beulich wrote:
> On 08.09.2020 17:08, Andrew Cooper wrote:
>> OSSTest has shown an assertion failure:
>> http://logs.test-lab.xenproject.org/osstest/logs/153906/test-xtf-amd64-amd64-1/serial-rimava1.log
>>
>> This is because we pass a non-NUL selector into svm_load_segs(), which is
>> something we must not do, as this path does not load the attributes/limit 
>> from
>> the GDT/LDT.
>>
>> Drop the {gs,gs}_sel parameters from svm_load_segs() and use 0 instead.  This
> Nit: {fs,gs}

Yeah - I noticed and fixed immediately after sending.  Also a plural
issue in the previous paragraph.

>
>> is fine even for non-zero NUL segments, as it is how the IRET instruction
>> behaves in all CPUs.
> To be honest, I'd not call it "fine", but "acceptable".

Ok.

> (And I don't consider IRET's behavior "fine" either.)

Nor me, but I'll happily use it to simplify our logic.

>> Only use the svm_load_segs() path when FS and GS are NUL, which is the common
>> case when scheduling a 64bit vcpu with 64bit userspace in context.
>>
>> Fixes: ad0fd291c5 ("x86/pv: Rewrite segment context switching from scratch")
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

Thanks.

~Andrew