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SubjectRE: [PATCH v13 06/12] KVM: x86: Add Intel PT virtualization work mode
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> > From: Chao Peng <chao.p.peng@linux.intel.com>
> >
> > Intel Processor Trace virtualization can be work in one of 2 possible
> > modes:
> >
> > a. System-Wide mode (default):
> > When the host configures Intel PT to collect trace packets
> > of the entire system, it can leave the relevant VMX controls
> > clear to allow VMX-specific packets to provide information
> > across VMX transitions.
> > KVM guest will not aware this feature in this mode and both
> > host and KVM guest trace will output to host buffer.
> >
> > b. Host-Guest mode:
> > Host can configure trace-packet generation while in
> > VMX non-root operation for guests and root operation
> > for native executing normally.
> > Intel PT will be exposed to KVM guest in this mode, and
> > the trace output to respective buffer of host and guest.
> > In this mode, tht status of PT will be saved and disabled
> > before VM-entry and restored after VM-exit if trace
> > a virtual machine.
> >
> > Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
> > Signed-off-by: Luwei Kang <luwei.kang@intel.com>
> > ---
>
> > +#define SECONDARY_EXEC_PT_USE_GPA 0x01000000
> > +#define VM_EXIT_CLEAR_IA32_RTIT_CTL 0x02000000
> > +#define VM_ENTRY_LOAD_IA32_RTIT_CTL 0x00040000
>
> Where are all of these bits documented? I'm looking at the latest SDM, volume 3 (325384-067US), and none of these bits aredocumented
> there.

This part is in the " Intel® Architecture Instruction Set Extensions and Future Features Programming Reference"
https://software.intel.com/sites/default/files/managed/c5/15/architecture-instruction-set-extensions-programming-reference.pdf

>
> > + GUEST_IA32_RTIT_CTL = 0x00002814,
> > + GUEST_IA32_RTIT_CTL_HIGH = 0x00002815,
>
> Where is this VMCS field documented?
>
> > +/* Default is SYSTEM mode. */
> > +static int __read_mostly pt_mode = PT_MODE_SYSTEM;
> > +module_param(pt_mode, int, S_IRUGO);
>
> As a module parameter, this doesn't allow much flexibility. Is it possible to make this decision per-VM, using a VM capability that can be set
> by userspace? (In that case, it may make sense to have a module parameter which allows/disallows the per-VM capability.)

It is a good idea from my point of view, I think it need more discussion and can be implement in next phase if have strong requirement.

>
>
> > +static inline bool cpu_has_vmx_intel_pt(void) {
> > + u64 vmx_msr;
> > +
> > + rdmsrl(MSR_IA32_VMX_MISC, vmx_msr);
> > + return !!(vmx_msr & MSR_IA32_VMX_MISC_INTEL_PT); }
>
> Instead of the rdmsr here, wouldn't it be better to cache the IA32_VMX_MISC MSR in vmcs_config?
> Nit: throughout this change, the '!!' isn't necessary when casting an integer type to bool.

MSR_IA32_VMX_MISC is not read frequency and just read once in this patch set during initialization.

Thanks,
Luwei Kang

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