Reviving an old thread here. On Wed, Jul 06, 2016 at 11:27:16PM +0200, Paolo Bonzini wrote: > On 06/07/2016 19:34, Eduardo Habkost wrote: > >> > Nothing is needed in the kernel actually. You can skip the intercept > >> > by running the guest with MSR_TSC_AUX set to the guest's expected value. > >> > Which KVM does, except that it's botched so I need to apply the > >> > patch in https://lkml.org/lkml/2016/4/13/802. > > Do you mean -cpu Opteron_G*,+rdtscp will be buggy on Linux v4.5? > > (v4.5 reports rdtscp as supported in GET_SUPPORTED_CPUID) > > > > Can we do something to make QEMU detect the buggy kernel before > > allowing rdtscp to be enabled, or should we just tell people to > > upgrade their kernel? > > We usually just tell people to use the latest stable kernel. > > Adding new CPU models is not a big deal, in fact it's almost easier than > getting compat properties right. :) Ok, can we finally revert 33b5e8c03ae7 ("target-i386: Disable rdtscp on Opteron_G* CPU models") in the qemu tree? Three years should be enough by now for 46896c73c1a4 ("KVM: svm: add support for RDTSCP") to have percolated downstream. Btw, its commit message talks about Linux not using RDTSCP but that will change soon. :-) Oh, and the EPYC qemu CPU model has CPUID_EXT2_RDTSCP and works just fine when patching in RDTSCP: [ 0.543197] apply_alternatives: feat: 3*32+18, old: (read_tsc+0x0/0x10 (ffffffff8101d1c0) len: 5), repl: (ffffffff824e6d33, len: 5), pad: 3 [ 0.544448] ffffffff8101d1c0: old_insn: 0f 31 90 90 90 [ 0.545023] ffffffff824e6d33: rpl_insn: 0f ae e8 0f 31 [ 0.545598] ffffffff8101d1c0: final_insn: 0f ae e8 0f 31 [ 0.546193] apply_alternatives: feat: 1*32+27, old: (read_tsc+0x0/0x10 (ffffffff8101d1c0) len: 5), repl: (ffffffff824e6d38, len: 3), pad: 3 [ 0.547195] ffffffff8101d1c0: old_insn: 0f ae e8 0f 31 [ 0.547775] ffffffff824e6d38: rpl_insn: 0f 01 f9 [ 0.548307] ffffffff8101d1c0: final_insn: 0f 01 f9 66 90 That final_insn which gets patched to by the alternatives is ffffffff8101d1c0: 0f 01 f9 rdtscp ffffffff8101d1c3: 66 90 xchg %ax,%ax Thx. -- Regards/Gruss, Boris. Good mailing practices for 400: avoid top-posting and trim the reply.
