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Re: [PATCH] xen: Introduce cmpxchg64() and guest_cmpxchg64()
Hi,
On 17/08/2020 11:33, Roger Pau Monné wrote:
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@xxxxxxxxxx>
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@xxxxxxxxxx>
Cc: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
---
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?
We have only 4 registers (r0 - r4) available for the arguments. With
64-bit value, we will be using 2 registers, some will end up to be
pushed on the stack.
This is assuming the compiler is not clever enough to see we are only
using the bottom 32-bit with some cmpxchg.
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; \
})
Unfortunately this can't compile if o and n are pointers because the
compiler will complain about the cast to uint64_t.
We would also need a cast when assigning to tmp because tmp may not be a
scalar type. This would lead to the same compiler issue.
The only way I could see to make it work would be to use the same trick
as we do for {read, write}_atomic() (see asm-arm/atomic.h). We are using
union and void pointer to prevent explicit cast.
But I am not sure whether the effort is really worth it.
Cheers,
--
Julien Grall
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