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Message-ID: <ZZXNXNZkCW8e1G5i@google.com>
Date: Wed, 3 Jan 2024 13:10:52 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Ashish Kalra <ashish.kalra@....com>
Cc: pbonzini@...hat.com, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
thomas.lendacky@....com, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
joro@...tes.org
Subject: Re: [PATCH] x86/sev: Add support for allowing zero SEV ASIDs.
On Wed, Jan 03, 2024, Ashish Kalra wrote:
> Hello Sean,
>
> On 1/2/2024 6:30 PM, Sean Christopherson wrote:
> > On Tue, Jan 02, 2024, Ashish Kalra wrote:
> > > @@ -2172,8 +2176,10 @@ void sev_vm_destroy(struct kvm *kvm)
> > > void __init sev_set_cpu_caps(void)
> > > {
> > > - if (!sev_enabled)
> > > + if (!sev_guests_enabled) {
> > Ugh, what a mess. The module param will show sev_enabled=false, but the caps
> > and CPUID will show SEV=true.
> >
> > And this is doubly silly because "sev_enabled" is never actually checked, e.g.
> > if misc cgroup support is disabled, KVM_SEV_INIT will try to reclaim ASIDs and
> > eventually fail with -EBUSY, which is super confusing to users.
>
> But this is what we expect that KVM_SEV_INIT will fail. In this case,
> sev_asid_new() will not actually try to reclaim any ASIDs as sev_misc_cg_try_charge()
> will fail before any ASID bitmap walking/reclamation and return an error which
> will eventually return -EBUSY to the user.
Please read what I wrote. "if misc cgroup support is disabled", i.e. if
CONFIG_CGROUP_MISC=n, then sev_misc_cg_try_charge() is a nop.
> > The other weirdness is that KVM can cause sev_enabled=false && sev_es_enabled=true,
> > but if *userspace* sets sev_enabled=false then sev_es_enabled is also forced off.
> But that is already the behavior without this patch applied.
> >
> > In other words, the least awful option seems to be to keep sev_enabled true :-(
> >
> > > kvm_cpu_cap_clear(X86_FEATURE_SEV);
> > > + return;
> > This is blatantly wrong, as it can result in KVM advertising SEV-ES if SEV is
> > disabled by the user.
> No, this ensures that we don't advertise any SEV capability if neither
> SEV/SEV-ES or in future SNP is enabled.
No, it does not. There is an early return statement here that prevents KVM from
invoking kvm_cpu_cap_clear() for X86_FEATURE_SEV_ES. Do I think userspace will
actually be tripped up by seeing SEV_ES without SEV? No. Is it unnecessarily
confusing? Yes.
> > > + }
> > > if (!sev_es_enabled)
> > > kvm_cpu_cap_clear(X86_FEATURE_SEV_ES);
> > > }
> > > @@ -2229,9 +2235,11 @@ void __init sev_hardware_setup(void)
> > > goto out;
> > > }
> > > - sev_asid_count = max_sev_asid - min_sev_asid + 1;
> > > - WARN_ON_ONCE(misc_cg_set_capacity(MISC_CG_RES_SEV, sev_asid_count));
> > > - sev_supported = true;
> > > + if (min_sev_asid <= max_sev_asid) {
> > > + sev_asid_count = max_sev_asid - min_sev_asid + 1;
> > > + WARN_ON_ONCE(misc_cg_set_capacity(MISC_CG_RES_SEV, sev_asid_count));
> > > + sev_supported = true;
> > > + }
> > > /* SEV-ES support requested? */
> > > if (!sev_es_enabled)
> > > @@ -2262,7 +2270,8 @@ void __init sev_hardware_setup(void)
> > > if (boot_cpu_has(X86_FEATURE_SEV))
> > > pr_info("SEV %s (ASIDs %u - %u)\n",
> > > sev_supported ? "enabled" : "disabled",
> > > - min_sev_asid, max_sev_asid);
> > > + sev_supported ? min_sev_asid : 0,
> > > + sev_supported ? max_sev_asid : 0);
> > I honestly think we should print the "garbage" values. The whole point of
> > printing the min/max SEV ASIDs was to help users understand why SEV is disabled,
> > i.e. printing zeroes is counterproductive.
> >
> > > if (boot_cpu_has(X86_FEATURE_SEV_ES))
> > > pr_info("SEV-ES %s (ASIDs %u - %u)\n",
> > > sev_es_supported ? "enabled" : "disabled",
> > It's all a bit gross, but I think we want something like this (I'm definitely
> > open to suggestions though):
> >
> > diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> > index d0c580607f00..bfac6d17462a 100644
> > --- a/arch/x86/kvm/svm/sev.c
> > +++ b/arch/x86/kvm/svm/sev.c
> > @@ -143,8 +143,20 @@ static void sev_misc_cg_uncharge(struct kvm_sev_info *sev)
> > static int sev_asid_new(struct kvm_sev_info *sev)
> > {
> > - int asid, min_asid, max_asid, ret;
> > + /*
> > + * SEV-enabled guests must use asid from min_sev_asid to max_sev_asid.
> > + * SEV-ES-enabled guest can use from 1 to min_sev_asid - 1. Note, the
> > + * min ASID can end up larger than the max if basic SEV support is
> > + * effectively disabled by disallowing use of ASIDs for SEV guests.
> > + */
> > + unsigned int min_asid = sev->es_active ? 1 : min_sev_asid;
> > + unsigned int max_asid = sev->es_active ? min_sev_asid - 1 : max_sev_asid;
> > + unsigned int asid;
> > bool retry = true;
> > + int ret;
> > +
> > + if (min_asid > max_asid)
> > + return -ENOTTY;
>
> This will still return -EBUSY to user.
Huh? The above is obviously -ENOTTY, and I don't see anything in the call stack
that will convert it to -EBUSY.
> This check here or the failure return from sev_misc_cg_try_charge() are quite
> similar in that sense.
>
> My point is that the same is achieved quite cleanly with
> sev_misc_cg_try_charge() too.
"Without additional effort" is not synonymous with "cleanly". Relying on an
accounting restriction that is completely orthogonal to basic functionality is
not "clean".
> > WARN_ON(sev->misc_cg);
> > sev->misc_cg = get_current_misc_cg();
> > @@ -157,12 +169,6 @@ static int sev_asid_new(struct kvm_sev_info *sev)
> > mutex_lock(&sev_bitmap_lock);
> > - /*
> > - * SEV-enabled guests must use asid from min_sev_asid to max_sev_asid.
> > - * SEV-ES-enabled guest can use from 1 to min_sev_asid - 1.
> > - */
> > - min_asid = sev->es_active ? 1 : min_sev_asid;
> > - max_asid = sev->es_active ? min_sev_asid - 1 : max_sev_asid;
> > again:
> > asid = find_next_zero_bit(sev_asid_bitmap, max_asid + 1, min_asid);
> > if (asid > max_asid) {
> > @@ -2232,8 +2238,10 @@ void __init sev_hardware_setup(void)
> > goto out;
> > }
> > - sev_asid_count = max_sev_asid - min_sev_asid + 1;
> > - WARN_ON_ONCE(misc_cg_set_capacity(MISC_CG_RES_SEV, sev_asid_count));
> > + if (min_sev_asid <= max_sev_asid) {
> > + sev_asid_count = max_sev_asid - min_sev_asid + 1;
> > + WARN_ON_ONCE(misc_cg_set_capacity(MISC_CG_RES_SEV, sev_asid_count));
> > + }
> > sev_supported = true;
> > /* SEV-ES support requested? */
> > @@ -2264,8 +2272,9 @@ void __init sev_hardware_setup(void)
> > out:
> > if (boot_cpu_has(X86_FEATURE_SEV))
> > pr_info("SEV %s (ASIDs %u - %u)\n",
> > - sev_supported ? "enabled" : "disabled",
> > - min_sev_asid, max_sev_asid);
> > + sev_supported ? (min_sev_asid <= max_sev_asid ? "enabled" : "unusable") : "disabled",
> > + sev_supported ? min_sev_asid : 0,
> > + sev_supported ? max_sev_asid : 0);
>
> We are not showing min and max ASIDs for SEV as {0,0} with this patch as
> sev_supported is true ?
Yes, and that is deliberate. See this from above:
: I honestly think we should print the "garbage" values. The whole point of
: printing the min/max SEV ASIDs was to help users understand why SEV is disabled,
: i.e. printing zeroes is counterproductive.
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