On Thu, Sep 27, 2012 at 08:31:22AM -0300, Marcelo Tosatti wrote: > On Thu, Sep 27, 2012 at 12:50:16AM +0000, Auld, Will wrote: > > Marcelo, > > > > I think I am missing something. There should be no needed changes to current algorithms that exist today. Does it seem that I have broken Zachary's implementation somehow? > > Yes. compute_guest_tsc() function must take ia32_tsc_adjust into > account. guest_read_tsc (and the SVM equivalent) also. Also, must take into account VMX->SVM migration. In that case, you should export IA32_TSC_ADJUST along with IA32_TSC MSR. Which brings us back to the initial question, if there are other means to provide stable TSC, why use this MSR? For example, VMWare guests have no need to use this MSR (because the hypervisor provides TSC guarantees). Then we come back to the two questions: - Is there anyone from Intel working on the Linux host side, where it makes sense to use this? - Are you sure its worthwhile to expose this to KVM guests? > > > > Thanks, > > > > Will > > > > -----Original Message----- > > From: Marcelo Tosatti [mailto:mtosatti@xxxxxxxxxx] > > Sent: Wednesday, September 26, 2012 5:29 PM > > To: Auld, Will > > Cc: kvm@xxxxxxxxxxxxxxx; Avi Kivity; Zhang, Xiantao; Liu, Jinsong > > Subject: Re: [PATCH] Enabling IA32_TSC_ADJUST for guest VM > > > > On Wed, Sep 26, 2012 at 10:58:46PM +0000, Auld, Will wrote: > > > Avi, Still working on your suggestions. > > > > > > Marcelo, > > > > > > The purpose is to be able to run guests that implement this change and not require they revert to the older method of adjusting the TSC. I am making no assumption about whether the guest checks to see if the times are good enough or just runs an algorithm every time but in any case this would allow the simpler, cleaner and less expensive algorithm to run if it exists. > > > > Will, you can choose to not expose the feature. Correct? > > > > Because this conflicts with the model that has been envisioned and developed by Zachary... for that model to continue to be functional you'll have to make sure the TSC emulation is adjusted accordingly to consider IA32_TSC_ADJUST (for example, when trapping TSC). > > > > >From that point of view, the patch below is incomplete. > > > > ... or KVM can choose to never expose the feature via CPUID and handle TSC consistency itself (i understand your perspective of getting a task complete, but unfortunately from my POV its not so simple). > > > > > Thanks, > > > > > > Will > > > > > > >The purpose of the IA32_TSC_ADJUST control is to make it easier for the operating system >(host) to decrease the delta between cores to an acceptable value, so that applications >can make use of direct RDTSC, correct? > > > > > > > >Why is it necessary for the guests to make use of such interface, if the hypervisor >could provide proper TSC? > > > > > > > >(not against exposing it to the guests, just thinking out loud). > > > > > > > >That is, if the purpose of the IA32_TSC_ADJUST is to provide proper synchronized TSC >across cores, and newer guests which should already make use of paravirt clock >interface, what is the point of exposing the feature? > > > > > > -----Original Message----- > > > From: Marcelo Tosatti [mailto:mtosatti@xxxxxxxxxx] > > > Sent: Wednesday, September 26, 2012 2:35 PM > > > To: Auld, Will > > > Cc: kvm@xxxxxxxxxxxxxxx; Avi Kivity; Zhang, Xiantao > > > Subject: Re: [PATCH] Enabling IA32_TSC_ADJUST for guest VM > > > > > > On Wed, Sep 19, 2012 at 05:44:46PM +0000, Auld, Will wrote: > > > > >From 9982bb73460b05c1328068aae047b14b2294e2da Mon Sep 17 00:00:00 > > > > >2001 > > > > From: Will Auld <will.auld@xxxxxxxxx> > > > > Date: Wed, 12 Sep 2012 18:10:56 -0700 > > > > Subject: [PATCH] Enabling IA32_TSC_ADJUST for guest VM > > > > > > > > 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 purpose of the IA32_TSC_ADJUST control is to make it easier for the operating system (host) to decrease the delta between cores to an acceptable value, so that applications can make use of direct RDTSC, correct? > > > > > > Why is it necessary for the guests to make use of such interface, if the hypervisor could provide proper TSC? > > > > > > (not against exposing it to the guests, just thinking out loud). > > > > > > That is, if the purpose of the IA32_TSC_ADJUST is to provide proper synchronized TSC across cores, and newer guests which should already make use of paravirt clock interface, what is the point of exposing the feature? > > > > > > > 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 perfor! > > > > ms 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. > > > > --- > > > > > > > arch/x86/include/asm/cpufeature.h | 1 + > > > > arch/x86/include/asm/kvm_host.h | 2 ++ > > > > arch/x86/include/asm/msr-index.h | 1 + > > > > arch/x86/kvm/cpuid.c | 4 ++-- > > > > arch/x86/kvm/vmx.c | 12 ++++++++++++ > > > > arch/x86/kvm/x86.c | 1 + > > > > 6 files changed, 19 insertions(+), 2 deletions(-) > > > > > > > > diff --git a/arch/x86/include/asm/cpufeature.h > > > > b/arch/x86/include/asm/cpufeature.h > > > > index 6b7ee5f..e574d81 100644 > > > > --- a/arch/x86/include/asm/cpufeature.h > > > > +++ b/arch/x86/include/asm/cpufeature.h > > > > @@ -199,6 +199,7 @@ > > > > > > > > /* 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 */ > > > > diff --git a/arch/x86/include/asm/kvm_host.h > > > > b/arch/x86/include/asm/kvm_host.h index 09155d6..8a001a4 100644 > > > > --- a/arch/x86/include/asm/kvm_host.h > > > > +++ b/arch/x86/include/asm/kvm_host.h > > > > @@ -442,6 +442,8 @@ struct kvm_vcpu_arch { > > > > u32 virtual_tsc_mult; > > > > u32 virtual_tsc_khz; > > > > > > > > + s64 tsc_adjust; > > > > + > > > > atomic_t nmi_queued; /* unprocessed asynchronous NMIs */ > > > > unsigned nmi_pending; /* NMI queued after currently running handler */ > > > > bool nmi_injected; /* Trying to inject an NMI this entry */ > > > > diff --git a/arch/x86/include/asm/msr-index.h > > > > b/arch/x86/include/asm/msr-index.h > > > > index 957ec87..8e82e29 100644 > > > > --- a/arch/x86/include/asm/msr-index.h > > > > +++ b/arch/x86/include/asm/msr-index.h > > > > @@ -231,6 +231,7 @@ > > > > #define MSR_IA32_EBL_CR_POWERON 0x0000002a > > > > #define MSR_EBC_FREQUENCY_ID 0x0000002c > > > > #define MSR_IA32_FEATURE_CONTROL 0x0000003a > > > > +#define MSR_TSC_ADJUST 0x0000003b > > > > > > > > #define FEATURE_CONTROL_LOCKED (1<<0) > > > > #define FEATURE_CONTROL_VMXON_ENABLED_INSIDE_SMX (1<<1) > > > > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c index > > > > 0595f13..8f5943e 100644 > > > > --- a/arch/x86/kvm/cpuid.c > > > > +++ b/arch/x86/kvm/cpuid.c > > > > @@ -248,8 +248,8 @@ static int do_cpuid_ent(struct kvm_cpuid_entry2 > > > > *entry, u32 function, > > > > > > > > /* cpuid 7.0.ebx */ > > > > const u32 kvm_supported_word9_x86_features = > > > > - F(FSGSBASE) | F(BMI1) | F(HLE) | F(AVX2) | F(SMEP) | > > > > - F(BMI2) | F(ERMS) | f_invpcid | F(RTM); > > > > + F(FSGSBASE) | F(TSC_ADJUST) | F(BMI1) | F(HLE) | > > > > + F(AVX2) | F(SMEP) | F(BMI2) | F(ERMS) | f_invpcid | F(RTM); > > > > > > > > /* all calls to cpuid_count() should be made on the same cpu */ > > > > get_cpu(); > > > > diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c index > > > > c00f03d..35d11b3 100644 > > > > --- a/arch/x86/kvm/vmx.c > > > > +++ b/arch/x86/kvm/vmx.c > > > > @@ -2173,6 +2173,9 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu, u32 msr_index, u64 *pdata) > > > > case MSR_IA32_SYSENTER_ESP: > > > > data = vmcs_readl(GUEST_SYSENTER_ESP); > > > > break; > > > > + case MSR_TSC_ADJUST: > > > > + data = (u64)vcpu->arch.tsc_adjust; > > > > + break; > > > > case MSR_TSC_AUX: > > > > if (!to_vmx(vcpu)->rdtscp_enabled) > > > > return 1; > > > > @@ -2241,6 +2244,13 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, u32 msr_index, u64 data) > > > > } > > > > ret = kvm_set_msr_common(vcpu, msr_index, data); > > > > break; > > > > + case MSR_TSC_ADJUST: > > > > +#define DUMMY 1 > > > > + vmx_adjust_tsc_offset(vcpu, > > > > + (s64)(data-vcpu->arch.tsc_adjust), > > > > + (bool)DUMMY); > > > > + vcpu->arch.tsc_adjust = (s64)data; > > > > + break; > > > > case MSR_TSC_AUX: > > > > if (!vmx->rdtscp_enabled) > > > > return 1; > > > > @@ -3931,6 +3941,8 @@ static int vmx_vcpu_reset(struct kvm_vcpu > > > > *vcpu) > > > > > > > > vcpu->arch.regs_avail = ~((1 << VCPU_REGS_RIP) | (1 << > > > > VCPU_REGS_RSP)); > > > > > > > > + vcpu->arch.tsc_adjust = 0x0; > > > > + > > > > vmx->rmode.vm86_active = 0; > > > > > > > > vmx->soft_vnmi_blocked = 0; > > > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index > > > > 42bce48..6c50f6c 100644 > > > > --- a/arch/x86/kvm/x86.c > > > > +++ b/arch/x86/kvm/x86.c > > > > @@ -824,6 +824,7 @@ static u32 msrs_to_save[] = { static unsigned > > > > num_msrs_to_save; > > > > > > > > static u32 emulated_msrs[] = { > > > > + MSR_TSC_ADJUST, > > > > MSR_IA32_TSCDEADLINE, > > > > MSR_IA32_MISC_ENABLE, > > > > MSR_IA32_MCG_STATUS, > > > > -- > > > > 1.7.1 > > > > > > > > -- > > > > To unsubscribe from this list: send the line "unsubscribe kvm" in > > > > the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo > > > > info at http://vger.kernel.org/majordomo-info.html > -- > To unsubscribe from this list: send the line "unsubscribe kvm" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
