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xen-devel
RE: [Xen-devel] Re: rdtsc hypercall, from userland?!? (was: rdtsc: corre
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To: |
Jan Beulich <JBeulich@xxxxxxxxxx>, Dan Magenheimer <dan.magenheimer@xxxxxxxxxx> |
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Subject: |
RE: [Xen-devel] Re: rdtsc hypercall, from userland?!? (was: rdtsc: correctness vs performance on Xen) |
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From: |
Ian Pratt <Ian.Pratt@xxxxxxxxxxxxx> |
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Date: |
Thu, 10 Sep 2009 11:14:02 +0100 |
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Cc: |
Ian, Jeremy Fitzhardinge <jeremy@xxxxxxxx>, "Xen-Devel \(E-mail\)" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Pratt <Ian.Pratt@xxxxxxxxxxxxx>, Keir Fraser <Keir.Fraser@xxxxxxxxxxxxx> |
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[Xen-devel] Re: rdtsc hypercall, from userland?!? (was: rdtsc: correctness vs performance on Xen) |
> 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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