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Date:   Tue, 22 Nov 2022 16:07:07 -0800
From:   "H. Peter Anvin" <hpa@zytor.com>
To:     Juergen Gross <jgross@suse.com>, Borislav Petkov <bp@alien8.de>
CC:     linux-kernel@vger.kernel.org, x86@kernel.org,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        Dave Hansen <dave.hansen@linux.intel.com>
Subject: =?US-ASCII?Q?Re=3A_=5BPATCH=5D_x86/boot=3A_skip_realmode_i?= =?US-ASCII?Q?nit_code_when_running_as_Xen_PV_guest?=
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On November 21, 2022 10:28:21 PM PST, Juergen Gross <jgross@suse=2Ecom> wro=
te:
>On 21=2E11=2E22 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=
=2E
>>>=20
>>> 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)=2E
>>>=20
>>> Skip the trampoline setup code for Xen PV guests=2E
>>>=20
>>> Fixes: 084ee1c641a0 ("x86, realmode: Relocator for realmode code")
>>> Signed-off-by: Juergen Gross <jgross@suse=2Ecom>
>>> ---
>>>   arch/x86/include/asm/realmode=2Eh | 4 ++--
>>>   arch/x86/realmode/init=2Ec        | 3 +++
>>>   2 files changed, 5 insertions(+), 2 deletions(-)
>>  =20
>>> diff --git a/arch/x86/include/asm/realmode=2Eh b/arch/x86/include/asm/=
realmode=2Eh
>>> index fd6f6e5b755a=2E=2E5bfce58f1bab 100644
>>> --- a/arch/x86/include/asm/realmode=2Eh
>>> +++ b/arch/x86/include/asm/realmode=2Eh
>>> @@ -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=2E */
>>> +	if (real_mode_header || cpu_feature_enabled(X86_FEATURE_XENPV))
>>> +		return 0;	/* already allocated or not needed=2E */
>>>     	return ALIGN(real_mode_blob_end - real_mode_blob, PAGE_SIZE);
>>>   }
>>> diff --git a/arch/x86/realmode/init=2Ec b/arch/x86/realmode/init=2Ec
>>> index 41d7669a97ad=2E=2E1826700b156e 100644
>>> --- a/arch/x86/realmode/init=2Ec
>>> +++ b/arch/x86/realmode/init=2Ec
>>> @@ -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
>>=20
>> This reminds me of the notorious if (xen) sprinkling from years ago=2E
>> Please don't do that=2E
>>=20
>
>Okay, what about plan B:
>
>- rework realmode/rm to:
>  + replace header=2ES with main=2Ec making it possible to initialize
>    struct real_mode_header using the struct definition
>  + optional: merge stack=2ES into main=2Ec
>- 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=2E It's called exactly twice per boot, it is hard to be less performanc=
e critical than that=2E