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Date: Wed, 1 Feb 2023 02:18:26 +0000
From: Sean Christopherson <seanjc@...gle.com>
To: Alexey Kardashevskiy <aik@....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 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).
Follow-up question: does KVM _have_ to wait until KVM_SEV_LAUNCH_UPDATE_VMSA to
set the flag?
> + 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.
> #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.
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?
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