[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] Re: [GIT PULL] xen /proc/mtrr implementation
* Gerd Hoffmann <kraxel@xxxxxxxxxx> wrote: >>>>> No, the linux kernel probably should do the wrmsr on one cpu only then. >>>> Why? > >> | The change of MTRR's on _any_ of the guest CPUs in a dom0 context >> | should immediately be refected on all CPUs. Assymetric MTRR >> | settings are madness. > > Exactly. And thats why it is pointless to let the dom0 kernel > write the mtrr msrs on more than one cpu. How does this negate my claim that the Linux kernel needs no modifications? (which i think your point is - let me know if you have some other point here.) the Xen hypervisor can simply repeat all requests (i.e. not care at all about the fact that a guest does these modifications on all CPUs it sees), or realize that the modification has already been done and skip it. This is in no way a performance sensitive codepath - mtrrs are only modified in init sequences - and setting mtrrs is slow and globally serialized to begin with. >>>>> Oops, the third "proper technical solutions" is missing. >>>> Yeah, the third one is to not touch MTRRs after bootup and use PAT. >>> Works only in case the CPU has PAT support. >> >> Which specific CPU without PAT support do you worry about? > > For example: I have a Notebook here, with MTRR and without PAT > according to the boot log. "Pentium III (Coppermine)" says > /proc/cpuinfo. That's a really old CPU, but even Coppermine has PAT support in the CPU. You need to go back to things like P5 200 MHz CPUs to find PAT-less CPUs. On the Linux side it's easy to enable it (and _such_ a patch would make sense indeed) - it's just that nobody has yet gone through the effort of testing and validating the PAT code on that CPU. If you care enough, you can do it, send a patch and ping the Intel folks about it. Once the issues are framed correctly, Linux is helped for real. Ingo _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxx http://lists.xensource.com/xen-devel
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