linux-kernel - Re: Getting weird TPM error after rebasing my tree to security/next-general

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Date:   Wed, 23 Jan 2019 07:26:42 +1300
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
Cc:     Jason Gunthorpe <jgg@...pe.ca>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>
Subject: Re: Getting weird TPM error after rebasing my tree to security/next-general

On Wed, Jan 23, 2019 at 2:29 AM Jarkko Sakkinen
<jarkko.sakkinen@...ux.intel.com> wrote:
>
> > >
> > > Fails on commit 170d13ca3a2fdaaa0283399247631b76b441cca2. Still works on
> > > preceding commit a959dc88f9c8900296ccf13e2f3e1cbc555a8917.
> >
> > This changes the IO access pattern in memcpy_to/fromio.. Presumably
> > CRB HW doesn't like the new 4 byte move? Swap each one in crb to
> > memcpy to confirm..
> >
> > If the HW requires particular access patterns you can't use
> > memcpy_to/fromio
>
> Did not have time to look at the commit at all but your deduction
> is correct. I know it without testing.
>
> Memory controller will feed 1's on unaligned read from IO memory,
> and as we can see from the TPM header, this change causes two of
> those:

Funky. But how did it work before then?

The new memcpy_fromio() is designed to have _predictable_ access
patterns. Not necessarily the best, but at least consistent.

Prevously, we used whatever random "memcpy()" implementation we
happened to pick, which *could* be aligned (particularly "rep movsb" -
absolutely horrible performance for MMIO, but by doing IO one byte at
a time it was certainly aligned ;), but most of our x86 memcpy
implementations don't actually try all that hard to align the source.
And the manual version will actually copy things *backwards* for some
cases.

Is it just that this particular hardware always happened to trigger
the ERMS case (ie "rep movsb")?

Anyway, Jason is correct that if a device has particular IO pattern
requirements, you shouldn't use "memcpy_fromio()" and friends, but
it's interesting how it apparently *happened* to work before.

             Linus