linux-kernel - Re: [PATCH] do not clean dummy variable in kexec path

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Date:   Mon, 5 Aug 2019 18:55:03 +0300
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Dave Young <dyoung@...hat.com>
Cc:     linux-efi <linux-efi@...r.kernel.org>,
        Kexec Mailing List <kexec@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Matthew Garrett <matthewgarrett@...gle.com>,
        Bhupesh Sharma <bhsharma@...hat.com>
Subject: Re: [PATCH] do not clean dummy variable in kexec path

On Mon, 5 Aug 2019 at 11:36, Dave Young <dyoung@...hat.com> wrote:
>
> kexec reboot fails randomly in UEFI based kvm guest.  The firmware
> just reset while calling efi_delete_dummy_variable();  Unfortunately
> I don't know how to debug the firmware, it is also possible a potential
> problem on real hardware as well although nobody reproduced it.
>
> The intention of efi_delete_dummy_variable is to trigger garbage collection
> when entering virtual mode.  But SetVirtualAddressMap can only run once
> for each physical reboot, thus kexec_enter_virtual_mode is not necessarily
> a good place to clean dummy object.
>

I would argue that this means it is not a good place to *create* the
dummy variable, and if we don't create it, we don't have to delete it
either.

> Drop efi_delete_dummy_variable so that kexec reboot can work.
>

Creating it and not deleting it is bad, so please try and see if we
can omit the creation on this code path instead.


> Signed-off-by: Dave Young <dyoung@...hat.com>
> ---
>  arch/x86/platform/efi/efi.c |    3 ---
>  1 file changed, 3 deletions(-)
>
> --- linux-x86.orig/arch/x86/platform/efi/efi.c
> +++ linux-x86/arch/x86/platform/efi/efi.c
> @@ -894,9 +894,6 @@ static void __init kexec_enter_virtual_m
>
>         if (efi_enabled(EFI_OLD_MEMMAP) && (__supported_pte_mask & _PAGE_NX))
>                 runtime_code_page_mkexec();
> -
> -       /* clean DUMMY object */
> -       efi_delete_dummy_variable();
>  #endif
>  }
>