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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Wed, 15 Nov 2023 15:07:24 -0500
Message-ID: <CAHk-=whrc-ruKs4Kt90EGzKd+pYhZFKs6bgBVCV=55BK+p1nzg@mail.gmail.com>
Subject: Re: [linus:master] [iov_iter] c9eec08bac: vm-scalability.throughput
 -16.9% regression
To:     Borislav Petkov <bp@alien8.de>
Cc:     David Howells <dhowells@redhat.com>,
        kernel test robot <oliver.sang@intel.com>,
        oe-lkp@lists.linux.dev, lkp@intel.com,
        linux-kernel@vger.kernel.org,
        Christian Brauner <brauner@kernel.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Jens Axboe <axboe@kernel.dk>, Christoph Hellwig <hch@lst.de>,
        Christian Brauner <christian@brauner.io>,
        Matthew Wilcox <willy@infradead.org>,
        David Laight <David.Laight@aculab.com>, ying.huang@intel.com,
        feng.tang@intel.com, fengwei.yin@intel.com
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On Wed, 15 Nov 2023 at 14:15, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> So my patch really is broken. It might happen to work when everything
> else goes right, and it's small, but it is very much a "proof of
> concept" rather than something that is actually acceptable.

More breakage details

 (a) need to not do this with the boot code and EFI stub code that
doesn't handle alternatives.

     It's not a huge deal in that obviously both alternatives work,
but it causes build issues:

      ld: error: unplaced orphan section `.altinstructions' from
`arch/x86/boot/compressed/misc.o'
      ld: error: unplaced orphan section `.altinstr_replacement' from
`arch/x86/boot/compressed/misc.o'
      ...

     etc

 - our current "memcpy_orig" fallback does unrolled copy loops, and
the rep_movs_alternative fallback obviously doesn't.

It's not clear that the unrolled copy loops matter for the in-kernel
kinds of copies, but who knows. The memcpy_orig code is definitely
trying to be smarter in some other ways too. So the fallback should
try a *bit* harder than I did, and not just with the whole "don't try
to handle exceptions" issue I mentioned.

              Linus