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Re: [PATCH 2/2] x86/trampoline: load the GDT located in the trampoline page
- To: Roger Pau Monne <roger.pau@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Tue, 2 May 2023 10:43:13 +0100
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- Cc: Jan Beulich <jbeulich@xxxxxxxx>, Wei Liu <wl@xxxxxxx>
- Delivery-date: Tue, 02 May 2023 09:43:37 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 02/05/2023 10:22 am, Roger Pau Monne wrote:
> When booting the BSP the portion of the code executed from the
> trampoline page will be using the GDT located in the hypervisor
> .text.head section rather than the GDT located in the trampoline page.
It's more subtle than this.
gdt_boot_descr references the trampoline GDT, but by it's position in
the main Xen image.
>
> If skip_realmode is not set the GDT located in the trampoline page
> will be loaded after having executed the BIOS call, otherwise the GDT
> from .text.head will be used for all the protected mode trampoline
> code execution.
>
> Note that both gdt_boot_descr and gdt_48 contain the same entries, but
> the former is located inside the hypervisor .text section, while the
> later lives in the relocated trampoline page.
>
> This is not harmful as-is, as both GDTs contain the same entries, but
> for consistency with the APs switch the BSP trampoline code to also
> use the GDT on the trampoline page.
>
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, although ...
> ---
> xen/arch/x86/boot/trampoline.S | 6 ++++++
> 1 file changed, 6 insertions(+)
>
> diff --git a/xen/arch/x86/boot/trampoline.S b/xen/arch/x86/boot/trampoline.S
> index cdecf949b410..e4b4b9091d0c 100644
> --- a/xen/arch/x86/boot/trampoline.S
> +++ b/xen/arch/x86/boot/trampoline.S
> @@ -164,6 +164,12 @@ GLOBAL(trampoline_cpu_started)
>
> .code32
> trampoline_boot_cpu_entry:
> + /*
> + * Load the GDT from the relocated trampoline page rather than the
> + * hypervisor .text section.
> + */
> + lgdt bootsym_rel(gdt_48, 4)
... I'd suggest rewording this to simply /* Switch to trampoline GDT */,
or perhaps with an "alias" in there somewhere.
The important point here is that we want to shed all pre-trampoline
state, and unexpectedly being on the wrong GDT alias certainly
complicated debugging this...
> +
> cmpb $0,bootsym_rel(skip_realmode,5)
> jnz .Lskip_realmode
>
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