RE: [Xen-devel] Re: rdtsc hypercall, from userland?!? (was: rdtsc: correctness vs performance on Xen)

[Ian Pratt <Ian.Pratt@xxxxxxxxxxxxx>]

> While possible, I'd suspect that the good performance you see for
> 64-bits wouldn't hold: You can't (without potential for ambiguity)
> re-use syscall for this purpose.

I'll bet all current 64b PV OSes use EAX as a simple system call number, so 
it's probably possible to do something hacky with negative values, after 
suitable auditing of current PV OSes and other common OSes. Not pretty, but I 
wouldn't throw the scheme out if an audit confirms the behaviour. 

Ian

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