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

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

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.

We've literally just loaded the GDT, at which point reloading all
segments *is* the expected thing to do.

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
 

It is a shame that the CR0 load breaks up the obvious connection with
lgdt, but IIRC, that was a consequence of how the code was laid out
previously.

Preferably with the above diff, Reviewed-by: Andrew Cooper
<andrew.cooper3@citrix.com>