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SubjectRe: [PATCH] x86/boot: skip realmode init code when running as Xen PV guest
On Mon, Nov 21, 2022 at 05:24:33PM +0100, Juergen Gross wrote:
> When running as a Xen PV guest there is no need for setting up the
> realmode trampoline, as realmode isn't supported in this environment.
>
> Trying to setup the trampoline has been proven to be problematic in
> some cases, especially when trying to debug early boot problems with
> Xen requiring to keep the EFI boot-services memory mapped (some
> firmware variants seem to claim basically all memory below 1M for boot
> services).
>
> Skip the trampoline setup code for Xen PV guests.
>
> Fixes: 084ee1c641a0 ("x86, realmode: Relocator for realmode code")
> Signed-off-by: Juergen Gross <jgross@suse.com>
> ---
> arch/x86/include/asm/realmode.h | 4 ++--
> arch/x86/realmode/init.c | 3 +++
> 2 files changed, 5 insertions(+), 2 deletions(-)

> diff --git a/arch/x86/include/asm/realmode.h b/arch/x86/include/asm/realmode.h
> index fd6f6e5b755a..5bfce58f1bab 100644
> --- a/arch/x86/include/asm/realmode.h
> +++ b/arch/x86/include/asm/realmode.h
> @@ -78,8 +78,8 @@ extern unsigned char secondary_startup_64_no_verify[];
>
> static inline size_t real_mode_size_needed(void)
> {
> - if (real_mode_header)
> - return 0; /* already allocated. */
> + if (real_mode_header || cpu_feature_enabled(X86_FEATURE_XENPV))
> + return 0; /* already allocated or not needed. */
>
> return ALIGN(real_mode_blob_end - real_mode_blob, PAGE_SIZE);
> }
> diff --git a/arch/x86/realmode/init.c b/arch/x86/realmode/init.c
> index 41d7669a97ad..1826700b156e 100644
> --- a/arch/x86/realmode/init.c
> +++ b/arch/x86/realmode/init.c
> @@ -202,6 +202,9 @@ static void __init set_real_mode_permissions(void)
>
> static int __init init_real_mode(void)
> {
> + if (cpu_feature_enabled(X86_FEATURE_XENPV))a

This reminds me of the notorious if (xen) sprinkling from years ago.
Please don't do that.

--
Regards/Gruss,
Boris.

https://people.kernel.org/tglx/notes-about-netiquette

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