Messages in this thread | | | Date | Tue, 07 Jul 2009 17:46:02 -0700 | From | "H. Peter Anvin" <> | Subject | Re: [PATCH] x86_64/__iowrite32_copy: don't use string move for PIO writes |
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John A. Gregor wrote: > > Anyway, we're very likely going to have to stick with asm for X86_64 no > matter how it gets implemented both for performance and as protection > against the compiler generating a rep movs sequence. > > So, I see at least a couple possible courses of action: > > 1. Redefine __iowrite32_copy(), document it better, and take the > new asm. > 2. Delete the .S file, warn about the possibility of the compiler > generating a rep movs sequence in the C code, and the ipath driver > will have a private copy routine again. >
I discussed this with H.J. Lu (x86 gcc maintainer), and he stated rather unequivocally that gcc will not generate string instructions for a volatile store, and that the gcc team considers that part of the definition for volatile. So the C version is guaranteed to be good; we should just remove the assembly version.
Redefining a public API -- no matter if you were the original authors -- is not an option. If you need something other than __iowrite32_copy() or __iowrite64_copy() you should define a new API or make it a private one. Either which way, a C implementation is guaranteed to be safe.
-hpa
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