Messages in this thread |  | | Date | Wed, 3 Jan 2024 14:41:39 -0600 | | Subject | Re: [PATCH] x86/sev: Add support for allowing zero SEV ASIDs. | | From | "Kalra, Ashish" <> |
| |
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.
> > 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. > >> + } >> 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. 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.
> 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 ?
Thanks, Ashish
> if (boot_cpu_has(X86_FEATURE_SEV_ES)) > pr_info("SEV-ES %s (ASIDs %u - %u)\n", > sev_es_supported ? "enabled" : "disabled",
|  |