Re: [PATCH][RFC] x86_64: Reload CS when startup_64 is used.

[Eric W. Biederman]

Magnus Damm <magnus@xxxxxxxxxxxxx> writes:

> On Mon, 2006-08-21 at 15:02 -0600, Eric W. Biederman wrote:
>> Magnus Damm <magnus@xxxxxxxxxxxxx> writes:
>> 
>> > x86_64: Reload CS when startup_64 is used.
>> >
>> > The current x86_64 startup code never reloads CS during the early boot
> process
>> > if the 64-bit function startup_64 is used as entry point. The 32-bit entry 
>> > point startup_32 does the right thing and reloads CS, and this is what most
>> > people are using if they use bzImage.
>> >
>> > This patch fixes the case when the Linux kernel is booted into using kexec
>> > under Xen. The Xen hypervisor is using large CS values which makes the
> x86_64
>> > kernel fail - but only if vmlinux is booted, bzImage works well because it
>> > is using the 32-bit entry point.
>> >
>> > The main question is if we require that the boot loader should setup CS
>> > to some certain offset to be able to boot the kernel. The sane solution IMO
>> > should be that the kernel requires that the loaded descriptors are correct,
>> > but that the exact offset within the GDT the boot loader is using should not
>> > matter. This is the way the i386 boot works if I understand things
> correctly.
>> 
>> What extra reload of cs does Xen introduce?
>
> None, but Xen is using CS values that are very different from Linux:
>
> xen/include/public/arch-x86_64.h:
>
> #define FLAT_RING3_CS32 0xe023  /* GDT index 260 */
> #define FLAT_RING3_CS64 0xe033  /* GDT index 261 */
> #define FLAT_RING3_DS32 0xe02b  /* GDT index 262 */
> #define FLAT_RING3_DS64 0x0000  /* NULL selector */
> #define FLAT_RING3_SS32 0xe02b  /* GDT index 262 */
> #define FLAT_RING3_SS64 0xe02b  /* GDT index 262 */
>
> The main problem is that startup_64 depends on that CS is set to
> __KERNEL_CS when it shouldn't. On top of that the purgatory code in
> kexec-tools doesn't setup CS when using a 64-bit entry point. The
> following (mangled) patch solves that for me:

I believe the cs shadow registers are fine.

> --- 0001/purgatory/arch/x86_64/entry64.S
> +++ work/purgatory/arch/x86_64/entry64.S        2006-08-18
> 15:34:23.000000000 +0900
> @@ -37,8 +37,9 @@ entry64:
>         movl    %eax, %fs
>         movl    %eax, %gs
>
> -       /* In 64bit mode the code segment is meaningless */

Would you care to explain to me how the above comment is not true.
As I recall the only meaning %cs has is if you are a kernel
or user space process.  The base address and everything else
mean nothing.

In addition the value in %cs never means anything.  It is the
values in the cs shadow registers that count.  The value in %cs
just reflects where those values in %cs came from.

So if we never reload %cs and only use the shadow values why does 
the value in %cs matter?

>> I'm not really comfortable with a half virtualized case.
>
> That I don't understand, care to explain more?

The only case where I can think of that the value in %cs would matter
is if you change %cs.  The only way I can see that is if you are
half para-virtualized.  Because we are running privileged
instructions in startup_64 it shouldn't work for the paravirtualized
case, and it should work just like it does today in the
fully virtualized case.



Eric

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