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Re: [Xen-devel] [PATCH 1/3] x86/vpmu: Calculate vpmu_enabled() based on vpmu_mode value
On 02/13/2017 07:50 AM, Jan Beulich wrote:
On 13.02.17 at 03:29, <boris.ostrovsky@xxxxxxxxxx> wrote:vpmu_enabled() is currently reported based on VPMU_CONTEXT_ALLOCATED bit of VPMU's flags. This presents a problem on Intel processors where VPMU context is allocated lazily, during the first access to VPMU MSRs. With such delayed allocation we, for example, cannot properly report CPUID's leaf 0xa since it is likely that the leaf is queried by a guest before the guest attempts ro read or write the MSR. Instead of keying vpmu_enabled() off the flags we should compute it based on vpmu_mode.Doesn't this have its own downsides? What if the mode changes behind the back of a DomU? While certain mode changes are disallowed while there are active users, there's nothing preventing mode or features from changing between a DomU querying CPUID and enabling the vPMU.
As you said, the mode can't change after a domU VCPU has been initialized, i.e. before the VCPU started running.
Of course, this was broken, which is what patch 2 (not quite successfully) tried to fix. And I proposed in my reply to Andrew
to instead define vpmu_enabled() as !!vcpu_vpmu(v)->arch_vpmu_ops. That should solve problems that both patches try to address. -boris
--- a/xen/arch/x86/cpu/vpmu.c +++ b/xen/arch/x86/cpu/vpmu.c @@ -110,6 +110,12 @@ static void __init parse_vpmu_params(char *s) printk("VPMU: unknown flags: %s - vpmu disabled!\n", s); } +bool_t vpmu_enabled (const struct vcpu *v)Stray blank. Jan
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