Messages in this thread | | | Date | Wed, 18 May 2011 13:22:59 +0200 | From | Ingo Molnar <> | Subject | Re: [RFC v3 0/8] x86, xsave: rework of extended state handling, LWP support |
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* Hans Rosenfeld <hans.rosenfeld@amd.com> wrote:
> > Here are a couple of suggestions to LWP hardware designers: > > > > - the fact that LWP cannot count kernel events right now is unfortunate - > > there's no reason not to allow privileged user-space to request ring 3 > > events as well - hopefully this misfeature will be fixed in future > > iterations of the hardware. > > > > - it would be nice to allow the per task masking/unmasking of LWP without > > having to modify the cr0 (which can be expensive). A third mode > > implemented in the LWP_CFG MSG would suffice: it would make the LWP > > instructions privileged, but would otherwise allow LWP event collection > > to occur even on sandboxed code. > > > > - it would be nice to also log the previous retired instruction in the > > trace entry, to ease decoding of the real instruction that generated > > an event. (Fused instructions can generate their RIP at the first > > instruction.) > > I will forward this to our hardware designers, but I have my doubts about the > first two of your suggestions. They seem to be orthogonal to what LWP is > supposed to be.
Not sure why you think those two suggestions are 'orthogonal to LWP', they are not:
- the second suggestion adds a third security model to the current all-or-nothing nature of LWP instructions.
- the first suggestion is a variation of its current security model as well: it allows LWP driven event collection in kernel mode, not just user mode.
There is nothing fundamentally ring-3-only about the concept of 'light weight profiling' - while ring-3-only event collection is understandably necessary for unprivileged user-space, it is not the only interesting mode of lightweight event collection.
Thanks,
Ingo
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