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

On 7/23/20 7:00 PM, Andrew Cooper wrote:
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@xxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- 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
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.

Hello Jan and Andrew,

Is there anything I can do to help with the delivery/merging of this patch?

If it would help, I can prepare and publish a patch according to Andrew's
comments. Given that the patch is not my work though, I assume that it
would be appropriate for me credit both of you in the commit message and
add a Signed-off-by tag in the commit message for each of you.




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