linux-kernel - Re: [PATCH 5/8] x86/mce: Avoid tail copy when machine check terminated a copy from user

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Message-ID: <20200917215751.GB16591@agluck-desk2.amr.corp.intel.com>
Date:   Thu, 17 Sep 2020 14:57:51 -0700
From:   "Luck, Tony" <tony.luck@...el.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Youquan Song <youquan.song@...el.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/8] x86/mce: Avoid tail copy when machine check
 terminated a copy from user

On Thu, Sep 17, 2020 at 07:04:06PM +0200, Borislav Petkov wrote:
> So actually, I'm thinking:
> 
> .LMCE_during_user_access:
> 	mov $-ENODEV, %eax
> 	ASM_CLAC
> 	ret
> 
> I have no clue which error code we should put there but it should be an
> error code which tells you not to retry and to back off immediately.

That does look a lot easier to understand *at this point* in the code.

But the existing iterator code is not expecting an error code.  Just a
count of bytes not copied.

So doing this would mean some surgery on the maze of giant #defines that
is lib/iov_iter.c

-Tony