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

["M. Vefa Bicakci" <m.v.b@xxxxxxxxxx>]

On 7/30/20 2:31 AM, Andrew Cooper wrote:
> On 30/07/2020 00:29, M. Vefa Bicakci wrote:
> > 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
> > > > > 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.
> > Hello Jan and Andrew,
> >
> > Is there anything I can do to help with the delivery/merging of this
> > patch?
> It was committed last Friday.
>
> https://xenbits.xen.org/gitweb/?p=xen.git;a=commitdiff;h=55f8c389d4348cc517946fdcb10794112458e81e
>
> I presume Jan will backport it to stable trees when he's not OoO.
Great -- thanks! (And sorry for not checking the git tree prior to
sending my e-mail.)

Vefa