Re: [PATCH v6 13/14] KVM: x86: Introduce new KVM_FEATURE_SEV_LIVE_MIGRATION feature & Custom MSR.

[Ashish Kalra]

Hello Brijesh,

On Mon, Mar 30, 2020 at 10:52:16AM -0500, Brijesh Singh wrote:
> 
> On 3/30/20 1:23 AM, Ashish Kalra wrote:
> > From: Ashish Kalra <ashish.kalra@xxxxxxx>
> >
> > Add new KVM_FEATURE_SEV_LIVE_MIGRATION feature for guest to check
> > for host-side support for SEV live migration. Also add a new custom
> > MSR_KVM_SEV_LIVE_MIG_EN for guest to enable the SEV live migration
> > feature.
> >
> > Also, ensure that _bss_decrypted section is marked as decrypted in the
> > page encryption bitmap.
> >
> > Signed-off-by: Ashish Kalra <ashish.kalra@xxxxxxx>
> > ---
> >  Documentation/virt/kvm/cpuid.rst     |  4 ++++
> >  Documentation/virt/kvm/msr.rst       | 10 ++++++++++
> >  arch/x86/include/asm/kvm_host.h      |  3 +++
> >  arch/x86/include/uapi/asm/kvm_para.h |  5 +++++
> >  arch/x86/kernel/kvm.c                |  4 ++++
> >  arch/x86/kvm/cpuid.c                 |  3 ++-
> >  arch/x86/kvm/svm.c                   |  5 +++++
> >  arch/x86/kvm/x86.c                   |  7 +++++++
> >  arch/x86/mm/mem_encrypt.c            | 14 +++++++++++++-
> >  9 files changed, 53 insertions(+), 2 deletions(-)
> 
> 
> IMHO, this patch should be broken into multiple patches as it touches
> guest, and hypervisor at the same time. The first patch can introduce
> the feature flag in the kvm, second patch can make the changes specific
> to svm,  and third patch can focus on how to make use of that feature
> inside the guest. Additionally invoking the HC to clear the
> __bss_decrypted section should be either squash in Patch 10/14 or be a
> separate patch itself.
> 
> 

Ok.

I will also move the __bss_decrypted section HC to a separate patch.

> > diff --git a/Documentation/virt/kvm/cpuid.rst b/Documentation/virt/kvm/cpuid.rst
> > index 01b081f6e7ea..fcb191bb3016 100644
> > --- a/Documentation/virt/kvm/cpuid.rst
> > +++ b/Documentation/virt/kvm/cpuid.rst
> > @@ -86,6 +86,10 @@ KVM_FEATURE_PV_SCHED_YIELD        13          guest checks this feature bit
> >                                                before using paravirtualized
> >                                                sched yield.
> >  
> > +KVM_FEATURE_SEV_LIVE_MIGRATION    14          guest checks this feature bit
> > +                                              before enabling SEV live
> > +                                              migration feature.
> > +
> >  KVM_FEATURE_CLOCSOURCE_STABLE_BIT 24          host will warn if no guest-side
> >                                                per-cpu warps are expeced in
> >                                                kvmclock
> > diff --git a/Documentation/virt/kvm/msr.rst b/Documentation/virt/kvm/msr.rst
> > index 33892036672d..7cd7786bbb03 100644
> > --- a/Documentation/virt/kvm/msr.rst
> > +++ b/Documentation/virt/kvm/msr.rst
> > @@ -319,3 +319,13 @@ data:
> >  
> >  	KVM guests can request the host not to poll on HLT, for example if
> >  	they are performing polling themselves.
> > +
> > +MSR_KVM_SEV_LIVE_MIG_EN:
> > +        0x4b564d06
> > +
> > +	Control SEV Live Migration features.
> > +
> > +data:
> > +        Bit 0 enables (1) or disables (0) host-side SEV Live Migration feature.
> > +        Bit 1 enables (1) or disables (0) support for SEV Live Migration extensions.
> > +        All other bits are reserved.
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index a96ef6338cd2..ad5faaed43c0 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -780,6 +780,9 @@ struct kvm_vcpu_arch {
> >  
> >  	u64 msr_kvm_poll_control;
> >  
> > +	/* SEV Live Migration MSR (AMD only) */
> > +	u64 msr_kvm_sev_live_migration_flag;
> > +
> >  	/*
> >  	 * Indicates the guest is trying to write a gfn that contains one or
> >  	 * more of the PTEs used to translate the write itself, i.e. the access
> > diff --git a/arch/x86/include/uapi/asm/kvm_para.h b/arch/x86/include/uapi/asm/kvm_para.h
> > index 2a8e0b6b9805..d9d4953b42ad 100644
> > --- a/arch/x86/include/uapi/asm/kvm_para.h
> > +++ b/arch/x86/include/uapi/asm/kvm_para.h
> > @@ -31,6 +31,7 @@
> >  #define KVM_FEATURE_PV_SEND_IPI	11
> >  #define KVM_FEATURE_POLL_CONTROL	12
> >  #define KVM_FEATURE_PV_SCHED_YIELD	13
> > +#define KVM_FEATURE_SEV_LIVE_MIGRATION	14
> >  
> >  #define KVM_HINTS_REALTIME      0
> >  
> > @@ -50,6 +51,7 @@
> >  #define MSR_KVM_STEAL_TIME  0x4b564d03
> >  #define MSR_KVM_PV_EOI_EN      0x4b564d04
> >  #define MSR_KVM_POLL_CONTROL	0x4b564d05
> > +#define MSR_KVM_SEV_LIVE_MIG_EN	0x4b564d06
> >  
> >  struct kvm_steal_time {
> >  	__u64 steal;
> > @@ -122,4 +124,7 @@ struct kvm_vcpu_pv_apf_data {
> >  #define KVM_PV_EOI_ENABLED KVM_PV_EOI_MASK
> >  #define KVM_PV_EOI_DISABLED 0x0
> >  
> > +#define KVM_SEV_LIVE_MIGRATION_ENABLED			(1 << 0)
> > +#define KVM_SEV_LIVE_MIGRATION_EXTENSIONS_SUPPORTED	(1 << 1)
> > +
> >  #endif /* _UAPI_ASM_X86_KVM_PARA_H */
> > diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> > index 6efe0410fb72..8fcee0b45231 100644
> > --- a/arch/x86/kernel/kvm.c
> > +++ b/arch/x86/kernel/kvm.c
> > @@ -418,6 +418,10 @@ static void __init sev_map_percpu_data(void)
> >  	if (!sev_active())
> >  		return;
> >  
> > +	if (kvm_para_has_feature(KVM_FEATURE_SEV_LIVE_MIGRATION)) {
> > +		wrmsrl(MSR_KVM_SEV_LIVE_MIG_EN, KVM_SEV_LIVE_MIGRATION_ENABLED);
> > +	}
> > +
> >  	for_each_possible_cpu(cpu) {
> >  		__set_percpu_decrypted(&per_cpu(apf_reason, cpu), sizeof(apf_reason));
> >  		__set_percpu_decrypted(&per_cpu(steal_time, cpu), sizeof(steal_time));
> > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> > index b1c469446b07..74c8b2a7270c 100644
> > --- a/arch/x86/kvm/cpuid.c
> > +++ b/arch/x86/kvm/cpuid.c
> > @@ -716,7 +716,8 @@ static inline int __do_cpuid_func(struct kvm_cpuid_entry2 *entry, u32 function,
> >  			     (1 << KVM_FEATURE_ASYNC_PF_VMEXIT) |
> >  			     (1 << KVM_FEATURE_PV_SEND_IPI) |
> >  			     (1 << KVM_FEATURE_POLL_CONTROL) |
> > -			     (1 << KVM_FEATURE_PV_SCHED_YIELD);
> > +			     (1 << KVM_FEATURE_PV_SCHED_YIELD) |
> > +			     (1 << KVM_FEATURE_SEV_LIVE_MIGRATION);
> 
> 
> Do we want to enable this feature unconditionally ? Who will clear the
> feature flags for the non-SEV guest ?
>

The guest only enables/activates this feature if sev is active.

> >  
> >  		if (sched_info_on())
> >  			entry->eax |= (1 << KVM_FEATURE_STEAL_TIME);
> > diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> > index c99b0207a443..60ddc242a133 100644
> > --- a/arch/x86/kvm/svm.c
> > +++ b/arch/x86/kvm/svm.c
> > @@ -7632,6 +7632,7 @@ static int svm_page_enc_status_hc(struct kvm *kvm, unsigned long gpa,
> >  				  unsigned long npages, unsigned long enc)
> >  {
> >  	struct kvm_sev_info *sev = &to_kvm_svm(kvm)->sev_info;
> > +	struct kvm_vcpu *vcpu = kvm->vcpus[0];
> >  	kvm_pfn_t pfn_start, pfn_end;
> >  	gfn_t gfn_start, gfn_end;
> >  	int ret;
> > @@ -7639,6 +7640,10 @@ static int svm_page_enc_status_hc(struct kvm *kvm, unsigned long gpa,
> >  	if (!sev_guest(kvm))
> >  		return -EINVAL;
> >  
> > +	if (!(vcpu->arch.msr_kvm_sev_live_migration_flag &
> > +		KVM_SEV_LIVE_MIGRATION_ENABLED))
> > +		return -ENOTTY;
> > +
> >  	if (!npages)
> >  		return 0;
> >  
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 2127ed937f53..82867b8798f8 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -2880,6 +2880,10 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> >  		vcpu->arch.msr_kvm_poll_control = data;
> >  		break;
> >  
> > +	case MSR_KVM_SEV_LIVE_MIG_EN:
> > +		vcpu->arch.msr_kvm_sev_live_migration_flag = data;
> > +		break;
> > +
> >  	case MSR_IA32_MCG_CTL:
> >  	case MSR_IA32_MCG_STATUS:
> >  	case MSR_IA32_MC0_CTL ... MSR_IA32_MCx_CTL(KVM_MAX_MCE_BANKS) - 1:
> > @@ -3126,6 +3130,9 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> >  	case MSR_KVM_POLL_CONTROL:
> >  		msr_info->data = vcpu->arch.msr_kvm_poll_control;
> >  		break;
> > +	case MSR_KVM_SEV_LIVE_MIG_EN:
> > +		msr_info->data = vcpu->arch.msr_kvm_sev_live_migration_flag;
> > +		break;
> >  	case MSR_IA32_P5_MC_ADDR:
> >  	case MSR_IA32_P5_MC_TYPE:
> >  	case MSR_IA32_MCG_CAP:
> > diff --git a/arch/x86/mm/mem_encrypt.c b/arch/x86/mm/mem_encrypt.c
> > index c9800fa811f6..f6a841494845 100644
> > --- a/arch/x86/mm/mem_encrypt.c
> > +++ b/arch/x86/mm/mem_encrypt.c
> > @@ -502,8 +502,20 @@ void __init mem_encrypt_init(void)
> >  	 * With SEV, we need to make a hypercall when page encryption state is
> >  	 * changed.
> >  	 */
> > -	if (sev_active())
> > +	if (sev_active()) {
> > +		unsigned long nr_pages;
> > +
> >  		pv_ops.mmu.page_encryption_changed = set_memory_enc_dec_hypercall;
> > +
> > +		/*
> > +		 * Ensure that _bss_decrypted section is marked as decrypted in the
> > +		 * page encryption bitmap.
> > +		 */
> > +		nr_pages = DIV_ROUND_UP(__end_bss_decrypted - __start_bss_decrypted,
> > +			PAGE_SIZE);
> > +		set_memory_enc_dec_hypercall((unsigned long)__start_bss_decrypted,
> > +			nr_pages, 0);
> > +	}
> 
> 
> Isn't this too late, should we be making hypercall at the same time we
> clear the encryption bit ?
> 
>

Actually this is being done somewhat lazily, after the guest enables/activates the live migration feature, it should be fine to do it
here or it can be moved into sev_map_percpu_data() where the first hypercalls are done, in both cases the __bss_decrypted section will
be marked before the live migration process is initiated.

> >  #endif
> >  
> >  	pr_info("AMD %s active\n",

Thanks,
Ashish