Re: [PATCH] x86/boot: skip realmode init code when running as Xen PV guest

[H. Peter Anvin]

On November 21, 2022 10:28:21 PM PST, Juergen Gross <jgross@xxxxxxxx> wrote:
>On 21.11.22 20:08, Borislav Petkov wrote:
>> 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@xxxxxxxx>
>>> ---
>>>   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.
>> 
>
>Okay, what about plan B:
>
>- rework realmode/rm to:
>  + replace header.S with main.c making it possible to initialize
>    struct real_mode_header using the struct definition
>  + optional: merge stack.S into main.c
>- include realmode/rm addresses needed outside of it in struct
>  real_mode_header
>- setup a dummy struct real_mode_header in Xen PV code removing the
>  need to skip init_real_mode(), but making it basically a nop
>
>Would you be fine with that?
>
>
>Juergen

I'm wondering if init_real_mode should not simply be part of the platform ops. It's called exactly twice per boot, it is hard to be less performance critical than that.