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Re: [Qemu-devel] [PATCH 00/10, v3] target-alpha improvements
From: |
Jamie Lokier |
Subject: |
Re: [Qemu-devel] [PATCH 00/10, v3] target-alpha improvements |
Date: |
Fri, 26 Mar 2010 01:52:52 +0000 |
User-agent: |
Mutt/1.5.13 (2006-08-11) |
Richard Henderson wrote:
> I don't see how any sort of emulation of cmpxchg/load-locked is working
> for any currently enabled nptl target. I think how I've approached
> handling load-locked for alpha is probably the easiest way. Slightly
> better would be if TCG had a (set of) cmpxchg opcodes, which would have
> the benefit of getting the virt->phys->host address (and segfault handling)
> more correct. I've sort of totally ignored the faulting for now.
I guess you are thinking to save the value loaded by load-locked, and
use it as the "old" for host cmpxchg at target's store-conditional?
I'm not sure if that will have the correct behaviour for all target
architectures, where _any_ value written by another CPU during an
ll/sc sequence will prevent the sequence from proceeding, including
CPU #0 CPU #1
x <- load-locked(A)
y <- load(B)
x+1 -> store(A)
y+1 -> store(B)
x -> store(A)
f(x,y) -> store-cond(A)
Unless I made a mistake, the above cannot store f(x,y+1) into A, for
any interleaving (assume strongly ordered memory or barriers), on
machines where any store by another CPU breaks the condition. But on
machines which implement store-cond by atomic-cmpxchg using the
load-locked value, f(x,y+1) can be stored.
It'll be fine when ll/sc are only used to provide single-word atomic
calculations, but I'm not sure those are the only uses to which they
are put by any code anywhere. E.g. if I remember rightly, there was
some discussion of a planned unusual ll/sc use on the linux-arm list,
which involved a second word, but the idea wasn't ever implemented.
-- Jamie
- Re: [Qemu-devel] [PATCH 09/10] target-alpha: Implement load-locked/store-conditional properly., (continued)
- Re: [Qemu-devel] [PATCH 09/10] target-alpha: Implement load-locked/store-conditional properly., Richard Henderson, 2010/03/25
- Re: [Qemu-devel] [PATCH 09/10] target-alpha: Implement load-locked/store-conditional properly., Nathan Froyd, 2010/03/25
- Re: [Qemu-devel] [PATCH 09/10] target-alpha: Implement load-locked/store-conditional properly., Richard Henderson, 2010/03/25
- Re: [Qemu-devel] [PATCH 09/10] target-alpha: Implement load-locked/store-conditional properly., Nathan Froyd, 2010/03/25
- Re: [Qemu-devel] [PATCH 09/10] target-alpha: Implement load-locked/store-conditional properly., Richard Henderson, 2010/03/25
- Re: [Qemu-devel] [PATCH 09/10] target-alpha: Implement load-locked/store-conditional properly., Blue Swirl, 2010/03/25
- Re: [Qemu-devel] [PATCH 09/10] target-alpha: Implement load-locked/store-conditional properly., Richard Henderson, 2010/03/25
- Re: [Qemu-devel] [PATCH 09/10] target-alpha: Implement load-locked/store-conditional properly., Jamie Lokier, 2010/03/25
- Re: [Qemu-devel] [PATCH 09/10] target-alpha: Implement load-locked/store-conditional properly., Jamie Lokier, 2010/03/25
[Qemu-devel] [PATCH 08/10] target-alpha: Emit goto_tb opcodes., Richard Henderson, 2010/03/24
Re: [Qemu-devel] [PATCH 00/10, v3] target-alpha improvements,
Jamie Lokier <=