linux-kernel - Re: [PATCH] x86/efistub: Don't try to print after ExitBootService()

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Date:   Thu, 12 Oct 2023 13:51:13 +0300
From:   Nikolay Borisov <nik.borisov@...e.com>
To:     kirill.shutemov@...ux.intel.com
Cc:     ardb@...nel.org, linux-efi@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/efistub: Don't try to print after ExitBootService()



On 12.10.23 г. 13:14 ч., kirill.shutemov@...ux.intel.com wrote:
> On Wed, Oct 11, 2023 at 10:25:28PM +0300, Nikolay Borisov wrote:
>> setup_e820() is executed after UEFI's ExitBootService has been called.
>> This causes the firmware to throw an exception because Console IO
>> protocol handler is supposed to work only during boot service
>> environment. As per UEFI 2.9, section 12.1:
>>
>>   "This protocol isused to handle input and output of text-based
>>   information intended for the system user during the operation of code
>>   in the boot services environment."
>>
>> Running a TDX guest with TDVF with unaccepted memory disabled results in
>> the following output:
> 
> Oh. My bad.
> 
> But there's other codepath that does the same. If setup_e820() fails with
> EFI_BUFFER_TOO_SMALL, efi_stub_entry() would try to print "exit_boot()
> failed\n".
> 
> I wouldner if it is feasible to hook up earlyprintk console into
> efi_printk() machinery for after ExitBootService() case? Silent boot
> failure is not the best UX.
> 


In my testing I was able to transpose setup_e820 and efi 
exit_boot_service by calling exit_boot_func before setup_e820 which 
ensures the various memory variables are populated. Is there any 
specific reason why ExitBootServices is called before setting up the 
e820 table? AFAIU this is an arbitrary choice?