On 02/02/2010 03:57 AM, Kurt Kiefer wrote:
Hi all,
This is a vague/general question. For some background: I have a reason
(control of IA32_PERF_GLOBAL_CTRL) for loading/saving MSRs on
VM-entry/exit. To get this to work correctly, I made changes to use
the conventional VMX MSR load areas of the VMCS for this particular
MSR. Works great.
Is there a particular reason why MSRs are currently loaded/saved
through KVM's unconventional facilities (vmx.c:save_msrs(),
vmx.c:load_msrs()), rather than through VM entry/exit MSR load regions
in the VMCS? I see that only long mode guests on x86_64 are effected
by this.
Any insight could be useful. Do you think MSR loading via VMCS would
be faster? Are there downsides to doing it one way or the other?
kvm doesn't switch msrs on every entry/exit. For example, the syscall
msrs are only used by the syscall/sysret instructions, so we only switch
them before returning to userspace, which happens much less frequently
than vmexits.
The PMU is used by the processor at all times, so it makes perfect sense
to use the vmx autoload/autosave regions for that.
--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.
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