On 1/3/2024 3:10 PM, Sean Christopherson wrote:
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.
Actually, sev_asid_new() returning failure to sev_guest_init() will
cause it to return -EBUSY to user.
Thanks, Ashish
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.
