Commit ba904635 authored by Will Auld's avatar Will Auld Committed by Marcelo Tosatti
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KVM: x86: Emulate IA32_TSC_ADJUST MSR 



CPUID.7.0.EBX[1]=1 indicates IA32_TSC_ADJUST MSR 0x3b is supported

Basic design is to emulate the MSR by allowing reads and writes to a guest
vcpu specific location to store the value of the emulated MSR while adding
the value to the vmcs tsc_offset. In this way the IA32_TSC_ADJUST value will
be included in all reads to the TSC MSR whether through rdmsr or rdtsc. This
is of course as long as the "use TSC counter offsetting" VM-execution control
is enabled as well as the IA32_TSC_ADJUST control.

However, because hardware will only return the TSC + IA32_TSC_ADJUST +
vmsc tsc_offset for a guest process when it does and rdtsc (with the correct
settings) the value of our virtualized IA32_TSC_ADJUST must be stored in one
of these three locations. The argument against storing it in the actual MSR
is performance. This is likely to be seldom used while the save/restore is
required on every transition. IA32_TSC_ADJUST was created as a way to solve
some issues with writing TSC itself so that is not an option either.

The remaining option, defined above as our solution has the problem of
returning incorrect vmcs tsc_offset values (unless we intercept and fix, not
done here) as mentioned above. However, more problematic is that storing the
data in vmcs tsc_offset will have a different semantic effect on the system
than does using the actual MSR. This is illustrated in the following example:

The hypervisor set the IA32_TSC_ADJUST, then the guest sets it and a guest
process performs a rdtsc. In this case the guest process will get
TSC + IA32_TSC_ADJUST_hyperviser + vmsc tsc_offset including
IA32_TSC_ADJUST_guest. While the total system semantics changed the semantics
as seen by the guest do not and hence this will not cause a problem.

Signed-off-by: default avatarWill Auld <will.auld@intel.com>
Signed-off-by: default avatarMarcelo Tosatti <mtosatti@redhat.com>
parent 8fe8ab46

arch/x86/include/asm/cpufeature.h

+1 −0
Original line number Diff line number Diff line
@@ -202,6 +202,7 @@ 


/* Intel-defined CPU features, CPUID level 0x00000007:0 (ebx), word 9 */

#define X86_FEATURE_FSGSBASE	(9*32+ 0) /* {RD/WR}{FS/GS}BASE instructions*/

#define X86_FEATURE_TSC_ADJUST	(9*32+ 1) /* TSC adjustment MSR 0x3b */

#define X86_FEATURE_BMI1	(9*32+ 3) /* 1st group bit manipulation extensions */

#define X86_FEATURE_HLE		(9*32+ 4) /* Hardware Lock Elision */

#define X86_FEATURE_AVX2	(9*32+ 5) /* AVX2 instructions */

arch/x86/include/asm/kvm_host.h

+2 −0
Original line number Diff line number Diff line
@@ -444,6 +444,7 @@ struct kvm_vcpu_arch { 
	s8 virtual_tsc_shift;

	u32 virtual_tsc_mult;

	u32 virtual_tsc_khz;

	s64 ia32_tsc_adjust_msr;



	atomic_t nmi_queued;  /* unprocessed asynchronous NMIs */

	unsigned nmi_pending; /* NMI queued after currently running handler */
@@ -711,6 +712,7 @@ struct kvm_x86_ops { 
	bool (*has_wbinvd_exit)(void);



	void (*set_tsc_khz)(struct kvm_vcpu *vcpu, u32 user_tsc_khz, bool scale);

	u64 (*read_tsc_offset)(struct kvm_vcpu *vcpu);

	void (*write_tsc_offset)(struct kvm_vcpu *vcpu, u64 offset);



	u64 (*compute_tsc_offset)(struct kvm_vcpu *vcpu, u64 target_tsc);

arch/x86/include/asm/msr-index.h

+1 −0
Original line number Diff line number Diff line
@@ -236,6 +236,7 @@ 
#define MSR_IA32_EBL_CR_POWERON		0x0000002a

#define MSR_EBC_FREQUENCY_ID		0x0000002c

#define MSR_IA32_FEATURE_CONTROL        0x0000003a

#define MSR_IA32_TSC_ADJUST             0x0000003b



#define FEATURE_CONTROL_LOCKED				(1<<0)

#define FEATURE_CONTROL_VMXON_ENABLED_INSIDE_SMX	(1<<1)

arch/x86/kvm/cpuid.c

+2 −0
Original line number Diff line number Diff line
@@ -320,6 +320,8 @@ static int do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function, 
		if (index == 0) {

			entry->ebx &= kvm_supported_word9_x86_features;

			cpuid_mask(&entry->ebx, 9);

			// TSC_ADJUST is emulated

			entry->ebx |= F(TSC_ADJUST);

		} else

			entry->ebx = 0;

		entry->eax = 0;

arch/x86/kvm/cpuid.h

+8 −0
Original line number Diff line number Diff line
@@ -28,6 +28,14 @@ static inline bool guest_cpuid_has_xsave(struct kvm_vcpu *vcpu) 
	return best && (best->ecx & bit(X86_FEATURE_XSAVE));

}



static inline bool guest_cpuid_has_tsc_adjust(struct kvm_vcpu *vcpu)

{

	struct kvm_cpuid_entry2 *best;



	best = kvm_find_cpuid_entry(vcpu, 7, 0);

	return best && (best->ebx & bit(X86_FEATURE_TSC_ADJUST));

}



static inline bool guest_cpuid_has_smep(struct kvm_vcpu *vcpu)

{

	struct kvm_cpuid_entry2 *best;

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