Messages in this thread | | | Subject | Re: [patch 13/26] Xen-paravirt_ops: Consistently wrap paravirt ops callsites to make them patchable | From | Rusty Russell <> | Date | Tue, 20 Mar 2007 11:01:45 +1100 |
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On Mon, 2007-03-19 at 11:38 -0700, Linus Torvalds wrote: > > On Mon, 19 Mar 2007, Eric W. Biederman wrote: > > > > True. You can use all of the call clobbered registers. > > Quite often, the biggest single win of inlining is not so much the code > size (although if done right, that will be smaller too), but the fact that > inlining DOES NOT CLOBBER AS MANY REGISTERS!
Thanks Linus.
*This* was the reason that the current hand-coded calls only clobber % eax. It was a compromise between native (no clobbers) and others (might need a reg).
Now, since we decided to allow paravirt_ops operations to be normal C (ie. the patching is optional and done late), we actually push and pop % ecx and %edx. This makes the call site 10 bytes long, which is a nice size for patching anyway (enough for a movl $0, <addr>, a-la lguest's cli, or movw $0, %gs:<addr> if we supported SMP).
The current 6 paravirt ops which are patched cover the vast majority of calls (until the Xen patches, then we need ~4 more?). Jeremy chose to expand patching to cover *all* paravirt ops, rather than just the new hot ones, and that's where we tipped over the ugliness threshold.
Cheers, Rusty.
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