linux-kernel - Re: [PATCH v2 2/4] xen-pcifront: Align address of flags to size of unsigned long

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Message-ID: <20191202182851.GB22528@agluck-desk2.amr.corp.intel.com>
Date:   Mon, 2 Dec 2019 10:28:51 -0800
From:   "Luck, Tony" <tony.luck@...el.com>
To:     Jürgen Groß <jgross@...e.com>
Cc:     Fenghua Yu <fenghua.yu@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        David Laight <David.Laight@...lab.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>
Subject: Re: [PATCH v2 2/4] xen-pcifront: Align address of flags to size of
 unsigned long

On Tue, Nov 26, 2019 at 11:02:13AM +0100, Jürgen Groß wrote:
> On 25.11.19 20:43, Fenghua Yu wrote:
> > The address of "flags" is passed to atomic bitops which require the
> > address is aligned to size of unsigned long.
> > 
> > Signed-off-by: Fenghua Yu <fenghua.yu@...el.com>
> > ---
> >   include/xen/interface/io/pciif.h | 7 +++++--
> >   1 file changed, 5 insertions(+), 2 deletions(-)
> > 
> > diff --git a/include/xen/interface/io/pciif.h b/include/xen/interface/io/pciif.h
> > index d9922ae36eb5..639d5fb484a3 100644
> > --- a/include/xen/interface/io/pciif.h
> > +++ b/include/xen/interface/io/pciif.h
> > @@ -103,8 +103,11 @@ struct xen_pcie_aer_op {
> >   	uint32_t devfn;
> >   };
> >   struct xen_pci_sharedinfo {
> > -	/* flags - XEN_PCIF_* */
> > -	uint32_t flags;
> > +	/* flags - XEN_PCIF_*. Force alignment for atomic bit operations. */
> > +	union {
> > +		uint32_t	flags;
> > +		unsigned long	flags_alignment;
> > +	};
> >   	struct xen_pci_op op;
> >   	struct xen_pcie_aer_op aer_op;
> >   };
> > 
> 
> NAK.
> 
> This is an interface definition for communication between Xen dom0 and
> guests via shared memory. It can't be changed.

Bother. There doesn't seem to be anything else in the xen_pci_sharedinfo
structure (or any of its sub-components) that would force the whole thing
to be any more strictly aligned than uint32_t

Oh ... but it looks like this is the only allocation:


        pdev->sh_info =
            (struct xen_pci_sharedinfo *)__get_free_page(GFP_KERNEL);

so "flags" is page aligned.

Fenghua: drop this patch, it isn't needed.

-Tony