Re: [PATCH] x86/S3: put data segment registers into known state upon resume

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

On 23/07/2020 16:19, Jan Beulich wrote:
> On 23.07.2020 16:40, Andrew Cooper wrote:
>> On 20/07/2020 16:20, Jan Beulich wrote:
>>> wakeup_32 sets %ds and %es to BOOT_DS, while leaving %fs at what
>>> wakeup_start did set it to, and %gs at whatever BIOS did load into it.
>>> All of this may end up confusing the first load_segments() to run on
>>> the BSP after resume, in particular allowing a non-nul selector value
>>> to be left in %fs.
>>>
>>> Alongside %ss, also put all other data segment registers into the same
>>> state that the boot and CPU bringup paths put them in.
>>>
>>> Reported-by: M. Vefa Bicakci <m.v.b@runbox.com>
>>> Signed-off-by: Jan Beulich <jbeulich@suse.com>
>>>
>>> --- a/xen/arch/x86/acpi/wakeup_prot.S
>>> +++ b/xen/arch/x86/acpi/wakeup_prot.S
>>> @@ -52,6 +52,16 @@ ENTRY(s3_resume)
>>>          mov     %eax, %ss
>>>          mov     saved_rsp(%rip), %rsp
>>>  
>>> +        /*
>>> +         * Also put other segment registers into known state, like would
>>> +         * be done on the boot path. This is in particular necessary for
>>> +         * the first load_segments() to work as intended.
>>> +         */
>> I don't think the comment is helpful, not least because it refers to a
>> broken behaviour in load_segemnts() which is soon going to change anyway.
> Well, I can drop it. I merely thought I'd be nice and comment my
> code once in a while (and the comment could be dropped / adjusted
> when load_segments() changes)...
>
>> We've literally just loaded the GDT, at which point reloading all
>> segments *is* the expected thing to do.
> In a way, unless some/all are assumed to already hold a nul selector.
>
>> I'd recommend that the diff be simply:
>>
>> diff --git a/xen/arch/x86/acpi/wakeup_prot.S
>> b/xen/arch/x86/acpi/wakeup_prot.S
>> index dcc7e2327d..a2c41c4f3f 100644
>> --- a/xen/arch/x86/acpi/wakeup_prot.S
>> +++ b/xen/arch/x86/acpi/wakeup_prot.S
>> @@ -49,6 +49,10 @@ ENTRY(s3_resume)
>>          mov     %rax, %cr0
>>  
>>          mov     $__HYPERVISOR_DS64, %eax
>> +        mov     %eax, %ds
>> +        mov     %eax, %es
>> +        mov     %eax, %fs
>> +        mov     %eax, %gs
>>          mov     %eax, %ss
>>          mov     saved_rsp(%rip), %rsp
> So I had specifically elected to not put the addition there, to make
> sure the stack would get established first. But seeing both Roger
> and you ask me to do otherwise - well, so be it then.

There is no IDT.  Any fault is will be triple, irrespective of the exact
code layout.

This sequence actually matches what we have in __high_start().

I don't think it is wise to write code which presumes that
__HYPERVISOR_DS64 is 0 (it happens to be, but could easily be 0xe010 as
well), or that the trampoline has fixed behaviours for the segments.

~Andrew