From: "Roger Pau Monné" <roger.pau@citrix.com>
To: Julien Grall <julien@xen.org>
Cc: <xen-devel@lists.xenproject.org>,
Julien Grall <jgrall@amazon.com>,
Stefano Stabellini <sstabellini@kernel.org>,
Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>,
Jan Beulich <jbeulich@suse.com>,
Andrew Cooper <andrew.cooper3@citrix.com>, Wei Liu <wl@xen.org>,
Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Subject: Re: [PATCH] xen: Introduce cmpxchg64() and guest_cmpxchg64()
Date: Mon, 17 Aug 2020 12:33:06 +0200 [thread overview]
Message-ID: <20200817103306.GA828@Air-de-Roger> (raw)
In-Reply-To: <b620dc46-7446-a440-5fd2-fd1cc7f8ffa7@xen.org>
On Mon, Aug 17, 2020 at 10:42:54AM +0100, Julien Grall wrote:
> Hi,
>
> On 17/08/2020 10:24, Roger Pau Monné wrote:
> > On Sat, Aug 15, 2020 at 06:21:43PM +0100, Julien Grall wrote:
> > > From: Julien Grall <jgrall@amazon.com>
> > >
> > > The IOREQ code is using cmpxchg() with 64-bit value. At the moment, this
> > > is x86 code, but there is plan to make it common.
> > >
> > > To cater 32-bit arch, introduce two new helpers to deal with 64-bit
> > > cmpxchg.
> > >
> > > The Arm 32-bit implementation of cmpxchg64() is based on the __cmpxchg64
> > > in Linux v5.8 (arch/arm/include/asm/cmpxchg.h).
> > >
> > > Signed-off-by: Julien Grall <jgrall@amazon.com>
> > > Cc: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
> > > ---
> > > diff --git a/xen/include/asm-x86/guest_atomics.h b/xen/include/asm-x86/guest_atomics.h
> > > index 029417c8ffc1..f4de9d3631ff 100644
> > > --- a/xen/include/asm-x86/guest_atomics.h
> > > +++ b/xen/include/asm-x86/guest_atomics.h
> > > @@ -20,6 +20,8 @@
> > > ((void)(d), test_and_change_bit(nr, p))
> > > #define guest_cmpxchg(d, ptr, o, n) ((void)(d), cmpxchg(ptr, o, n))
> > > +#define guest_cmpxchg64(d, ptr, o, n) ((void)(d), cmpxchg64(ptr, o, n))
> > > +
> > > #endif /* _X86_GUEST_ATOMICS_H */
> > > /*
> > > diff --git a/xen/include/asm-x86/x86_64/system.h b/xen/include/asm-x86/x86_64/system.h
> > > index f471859c19cc..c1b16105e9f2 100644
> > > --- a/xen/include/asm-x86/x86_64/system.h
> > > +++ b/xen/include/asm-x86/x86_64/system.h
> > > @@ -5,6 +5,8 @@
> > > ((__typeof__(*(ptr)))__cmpxchg((ptr),(unsigned long)(o), \
> > > (unsigned long)(n),sizeof(*(ptr))))
> > > +#define cmpxchg64(ptr, o, n) cmpxchg(ptr, o, n)
> >
> > Why do you need to introduce an explicitly sized version of cmpxchg
> > for 64bit values?
> >
> > There's no cmpxchg{8,16,32}, so I would expect cmpxchg64 to just be
> > handled by cmpxchg detecting the size of the parameter passed to the
> > function.
> That works quite well for 64-bit arches. However, for 32-bit, you would need
> to take some detour so 32-bit and 64-bit can cohabit (you cannot simply
> replace unsigned long with uint64_t).
Oh, I see. Switching __cmpxchg on Arm 32 to use unsigned long long or
uint64_t would be bad, as you would then need two registers to pass
the value to the function, or push it on the stack?
Maybe do something like:
#define cmpxchg(ptr,o,n) ({ \
typeof(*(ptr)) tmp; \
\
switch ( sizeof(*(ptr)) ) \
{ \
case 8: \
tmp = __cmpxchg_mb64((ptr), (uint64_t)(o), \
(uint64_t)(n), sizeof(*(ptr)))) \
break; \
default: \
tmp = __cmpxchg_mb((ptr), (unsigned long)(o), \
(unsigned long)(n), sizeof(*(ptr)))) \
break; \
} \
tmp; \
})
Roger.
next prev parent reply other threads:[~2020-08-17 10:33 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-15 17:21 [PATCH] xen: Introduce cmpxchg64() and guest_cmpxchg64() Julien Grall
2020-08-16 19:26 ` Oleksandr
2020-08-17 9:24 ` Roger Pau Monné
2020-08-17 9:42 ` Julien Grall
2020-08-17 10:33 ` Roger Pau Monné [this message]
2020-08-17 11:05 ` Julien Grall
2020-08-17 11:50 ` Roger Pau Monné
2020-08-17 13:03 ` Julien Grall
2020-08-17 14:20 ` Roger Pau Monné
2020-08-19 9:22 ` Jan Beulich
2020-08-20 9:14 ` Julien Grall
2020-08-20 9:25 ` Jan Beulich
2020-08-20 9:34 ` Julien Grall
2020-08-19 9:18 ` Jan Beulich
2020-08-17 22:56 ` Stefano Stabellini
2020-08-18 10:30 ` Julien Grall
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