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

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Date:   Mon, 2 Dec 2019 14:29:02 -0800
From:   Fenghua Yu <fenghua.yu@...el.com>
To:     Jürgen Groß <jgross@...e.com>
Cc:     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>,
        Tony Luck <tony.luck@...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.
> 
> BTW: you should Cc: the maintainers for the files you are modifying.
> 
> 
> Juergen

After spending more time on the patch, actually I find this patch is not 
needed because there is no split lock issue on the "flags" field:

The bit offsets in all atomic bitops on the "flags" field are always 
constant values. On x86, an atomic bitop is optimized to use a locked byte
instruciton which only operates the byte containing the bit. So the
byte won't across cache line boundaries and there won't be split lock
issue in the atomic bitops

I will remove this patch in the next version of the patch set.

Thanks.

-Fenghua