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Date: Fri, 3 Feb 2023 14:37:28 +1100
From: Alexey Kardashevskiy <aik@....com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: kvm@...r.kernel.org, x86@...nel.org, linux-kernel@...r.kernel.org,
Yury Norov <yury.norov@...il.com>,
Venu Busireddy <venu.busireddy@...cle.com>,
Tony Luck <tony.luck@...el.com>,
Tom Lendacky <thomas.lendacky@....com>,
Thomas Gleixner <tglx@...utronix.de>,
Sandipan Das <sandipan.das@....com>,
Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
Paolo Bonzini <pbonzini@...hat.com>,
Michael Roth <michael.roth@....com>,
Mario Limonciello <mario.limonciello@....com>,
Kim Phillips <kim.phillips@....com>,
Kees Cook <keescook@...omium.org>,
Juergen Gross <jgross@...e.com>,
Jakub Kicinski <kuba@...nel.org>,
Ingo Molnar <mingo@...hat.com>,
Dave Hansen <dave.hansen@...ux.intel.com>,
Daniel Sneddon <daniel.sneddon@...ux.intel.com>,
Brijesh Singh <brijesh.singh@....com>,
Borislav Petkov <bp@...en8.de>,
Arnaldo Carvalho de Melo <acme@...hat.com>,
Andrew Cooper <andrew.cooper3@...rix.com>,
Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
Adrian Hunter <adrian.hunter@...el.com>,
"Peter Zijlstra (Intel)" <peterz@...radead.org>,
"Jason A. Donenfeld" <Jason@...c4.com>,
"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH kernel v3 2/3] KVM: SEV: Enable data breakpoints in SEV-ES
On 01/02/2023 13:18, Sean Christopherson wrote:
> On Fri, Jan 20, 2023, Alexey Kardashevskiy wrote:
>> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
>> index 4826e6cc611b..61f2cad1cbaf 100644
>> --- a/arch/x86/kvm/svm/svm.h
>> +++ b/arch/x86/kvm/svm/svm.h
>> @@ -389,6 +389,8 @@ static inline bool vmcb12_is_intercept(struct vmcb_ctrl_area_cached *control, u3
>> return test_bit(bit, (unsigned long *)&control->intercepts);
>> }
>>
>> +extern bool sev_es_is_debug_swap_enabled(void);
>> +
>> static inline void set_dr_intercepts(struct vcpu_svm *svm)
>> {
>> struct vmcb *vmcb = svm->vmcb01.ptr;
>> @@ -410,8 +412,10 @@ static inline void set_dr_intercepts(struct vcpu_svm *svm)
>> vmcb_set_intercept(&vmcb->control, INTERCEPT_DR6_WRITE);
>> }
>>
>> - vmcb_set_intercept(&vmcb->control, INTERCEPT_DR7_READ);
>> - vmcb_set_intercept(&vmcb->control, INTERCEPT_DR7_WRITE);
>> + if (!sev_es_guest(svm->vcpu.kvm) || !sev_es_is_debug_swap_enabled()) {
>
> Looking below, doesn't this do the wrong thing if set_dr_intercepts() is called
> before SVM_SEV_FEAT_DEBUG_SWAP is set? I.e. when this is called before LAUNCH_UPDATE?
> Seems like this should check SVM_SEV_FEAT_DEBUG_SWAP in sev_features regardless
> of when SVM_SEV_FEAT_DEBUG_SWAP is set.
>
> And if KVM checks sev_features, then I _think_ we can avoid having to expose
> sev_es_debug_swap_enabled to svm.{c,h} (though why on earth {set,clr}_dr_intercepts()
> is in svm.h is another question for the future).
883b0a91f41a ("KVM: SVM: Move Nested SVM Implementation to nested.c")
did that. Makes sense for things like vmcb_set_intercept() but
{set,clr}_dr_intercepts() are still only called from svm.c so I'll move
them there (btw do I need a separate patch for that? usually yes)
>
> Follow-up question: does KVM _have_ to wait until KVM_SEV_LAUNCH_UPDATE_VMSA to
> set the flag?
Nope. Will repost soon as a reply to this mail.
>> + vmcb_set_intercept(&vmcb->control, INTERCEPT_DR7_READ);
>> + vmcb_set_intercept(&vmcb->control, INTERCEPT_DR7_WRITE);
>> + }
>>
>> recalc_intercepts(svm);
>> }
>> @@ -422,8 +426,12 @@ static inline void clr_dr_intercepts(struct vcpu_svm *svm)
>>
>> vmcb->control.intercepts[INTERCEPT_DR] = 0;
>>
>> - /* DR7 access must remain intercepted for an SEV-ES guest */
>> - if (sev_es_guest(svm->vcpu.kvm)) {
>> + /*
>> + * DR7 access must remain intercepted for an SEV-ES guest unless DebugSwap
>> + * (depends on NO_NESTED_DATA_BP) is enabled as otherwise a VM writing to DR7
>> + * from the #DB handler may trigger infinite loop of #DB's.
>> + */
>> + if (sev_es_guest(svm->vcpu.kvm) && !sev_es_is_debug_swap_enabled()) {
>> vmcb_set_intercept(&vmcb->control, INTERCEPT_DR7_READ);
>> vmcb_set_intercept(&vmcb->control, INTERCEPT_DR7_WRITE);
>> }
>>
>> @@ -52,11 +53,21 @@ module_param_named(sev, sev_enabled, bool, 0444);
>> /* enable/disable SEV-ES support */
>> static bool sev_es_enabled = true;
>> module_param_named(sev_es, sev_es_enabled, bool, 0444);
>> +
>> +/* enable/disable SEV-ES DebugSwap support */
>> +static bool sev_es_debug_swap_enabled = true;
>> +module_param_named(debug_swap, sev_es_debug_swap_enabled, bool, 0644);
>
> Module param needs 0444 permissions, i.e. shouldn't be writable after KVM is
> loaded. Though I don't know that providing a module param is warranted in this
> case.
> KVM provides module params for SEV and SEV-ES because there are legitimate
> reasons to turn them off, but at a glance, I don't see why we'd want that for this
> feature.
/me confused. You suggested this in the first place for (I think) for
the case if the feature is found to be broken later on so admins can
disable it.
And I was using it to verify "x86/debug: Fix stack recursion caused by
DR7 accesses" which is convenient but it is a minor thing.
>> #else
>> #define sev_enabled false
>> #define sev_es_enabled false
>> +#define sev_es_debug_swap false
>
> This needs to be sev_es_debug_swap_enabled, otherwise things fall apart with
> CONFIG_KVM_AMD_SEV=n.
>
> arch/x86/kvm/svm/sev.c: In function ‘sev_es_is_debug_swap_enabled’:
> arch/x86/kvm/svm/sev.c:69:16: error: ‘sev_es_debug_swap_enabled’ undeclared (first use in this function); did you mean ‘sev_es_is_debug_swap_enabled’?
> 69 | return sev_es_debug_swap_enabled;
> | ^~~~~~~~~~~~~~~~~~~~~~~~~
> | sev_es_is_debug_swap_enabled
>
>
>> #endif /* CONFIG_KVM_AMD_SEV */
>>
>> +bool sev_es_is_debug_swap_enabled(void)
>> +{
>> + return sev_es_debug_swap_enabled;
>> +}
>
> ...
>
>> @@ -604,6 +615,9 @@ static int sev_es_sync_vmsa(struct vcpu_svm *svm)
>> save->xss = svm->vcpu.arch.ia32_xss;
>> save->dr6 = svm->vcpu.arch.dr6;
>>
>> + if (sev_es_is_debug_swap_enabled())
>> + save->sev_features |= SVM_SEV_FEAT_DEBUG_SWAP;
>> +
>> pr_debug("Virtual Machine Save Area (VMSA):\n");
>> print_hex_dump_debug("", DUMP_PREFIX_NONE, 16, 1, save, sizeof(*save), false);
>>
>> @@ -2249,6 +2263,9 @@ void __init sev_hardware_setup(void)
>> out:
>> sev_enabled = sev_supported;
>> sev_es_enabled = sev_es_supported;
>> + if (sev_es_debug_swap_enabled)
>> + sev_es_debug_swap_enabled = sev_es_enabled &&
>> + cpu_feature_enabled(X86_FEATURE_NO_NESTED_DATA_BP);
>
> Slight preference for:
>
> if (!sev_es_enabled || !cpu_feature_enabled(X86_FEATURE_NO_NESTED_DATA_BP))
> sev_es_debug_swap_enabled = false;
>
> KVM does short-circuit some checks on module param values, but usually only to
> avoid additional setup.
>
>> #endif
>> }
>>
>> @@ -3027,6 +3044,18 @@ void sev_es_prepare_switch_to_guest(struct sev_es_save_area *hostsa)
>>
>> /* MSR_IA32_XSS is restored on VMEXIT, save the currnet host value */
>> hostsa->xss = host_xss;
>> +
>> + /* The DebugSwap SEV feature does Type B swaps of DR[0-3] */
>> + if (sev_es_is_debug_swap_enabled()) {
>> + hostsa->dr0 = native_get_debugreg(0);
>> + hostsa->dr1 = native_get_debugreg(1);
>> + hostsa->dr2 = native_get_debugreg(2);
>> + hostsa->dr3 = native_get_debugreg(3);
>> + hostsa->dr0_addr_mask = amd_get_dr_addr_mask(0);
>> + hostsa->dr1_addr_mask = amd_get_dr_addr_mask(1);
>> + hostsa->dr2_addr_mask = amd_get_dr_addr_mask(2);
>> + hostsa->dr3_addr_mask = amd_get_dr_addr_mask(3);
>> + }
>> }
>>
>> void sev_vcpu_deliver_sipi_vector(struct kvm_vcpu *vcpu, u8 vector)
>> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
>> index 60c7c880266b..6c54a3c9d442 100644
>> --- a/arch/x86/kvm/svm/svm.c
>> +++ b/arch/x86/kvm/svm/svm.c
>> @@ -1190,7 +1190,8 @@ static void init_vmcb(struct kvm_vcpu *vcpu)
>> set_exception_intercept(svm, UD_VECTOR);
>> set_exception_intercept(svm, MC_VECTOR);
>> set_exception_intercept(svm, AC_VECTOR);
>> - set_exception_intercept(svm, DB_VECTOR);
>> + if (!sev_es_is_debug_swap_enabled())
>> + set_exception_intercept(svm, DB_VECTOR);
>
> This is wrong. KVM needs to intercept #DBs when debugging non-SEV-ES VMs.
Sorry, not following. The #DB intercept for non-SEV-ES is enabled here.
Thanks,
> This _could_ be tied to X86_FEATURE_NO_NESTED_DATA_BP, but the KVM would need to
> toggle the intercept depending on whether or not userspace wants to debug the
> guest.
>
> Similar to the DR7 interception, can this check sev_features directly?
--
Alexey
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