On Tue, Jan 10, 2023, Paolo Bonzini wrote:
On 1/10/23 13:16, Emanuele Giuseppe Esposito wrote:
I think the test in patch 2 I wrote gives a better idea on what I am trying to fix:
Tests and descriptions of expected s vs. actual behavior explain what _you_ think should happen, but don't help explain what higher level bug is being fixed. IIUC, QEMU emulates RESET and expects the xAPIC ID to be re-initialized. That's the info that should be provided in the changelog since it ties KVM behavior to a real world userspace emulating actual hardware behavior.
if we are transitioning from x2APIC to xAPIC (RESET I would say, even though I am not sure if userspace really does it in the way I do it in the test, ie through KVM_SET_MSRS), the APIC_ID is not updated back in the right bits, and we can see that by querying the ID with KVM_GET_LAPIC after disabling x2APIC.
Now, if the way I reproduce this issue is correct, it is indeed a bug and needs to be fixed with the fix in patch 1 or something similar. I think it won't really make any difference if instead following what the doc says (x2APIC -> disabled -> xAPIC) we directly do x2APIC -> xAPIC.
Yes, the default value at reset is xAPIC mode, so a reset will do a KVM_SET_MSRS that clears X2APIC_ENABLE but leaves MSR_IA32_APICBASE_ENABLE set.
So, if I understand correctly...
The test in patch 2 started being developed to test ef40757743b47 ("KVM: x86: fix APICv/x2AVIC disabled when vm reboot by itself") even though I honestly didn't really understand how to replicate that bug (see cover letter) and instead I found this other possibility that still manages to screw APIC_ID.
... what you're saying is that there were two different bugs, but one fixing any one of them was enough to prevent the symptoms shown by commit ef40757743b47? That is:
- the APICv inhibit was set by KVM_GET_LAPIC because it called
kvm_lapic_xapic_id_updated(), and the call was unnecessary as fixed in commit ef40757743b47;
- however, there is no reason for the vCPU ID to be mismatched. It
happened because the code didn't handle the host-initiated x2APIC->xAPIC case and thus lacked a call to kvm_apic_set_xapic_id().
If so, I think the idea of the patch is fine.
Just one thing: your patch also changes the APIC_ID on the x2APIC->disabled transition, not just the "forbidden" (i.e. host- initiated only) x2APIC->xAPIC transition. I think this is okay too: the manual says:
10.4.3 Enabling or Disabling the Local APIC
When IA32_APIC_BASE[11] is set to 0, prior initialization to the APIC may be lost and the APIC may return to the state described in Section 10.4.7.1, “Local APIC State After Power-Up or Reset.”
10.4.7.1 Local APIC State After Power-Up or Reset
... The local APIC ID register is set to a unique APIC ID. ...
(which must be an xAPIC ID) and this is what your patch does.
Ugh, couldn't find that yesterday. It's actually irrelevant though, KVM already stuffs the APIC ID when the APIC goes from DISABLED to ENABLED (xAPIC) since commit:
49bd29ba1dbd ("KVM: x86: reset APIC ID when enabling LAPIC")
For giggles, and because I misread that like 5 times, I tested on hardware. Intel CPUs since at least Haswell make the APIC ID read-only, i.e. it's a moot point on Intel these days. But on AMD, the APIC ID is preserved across disabling => enabling xAPIC.
In fact perhaps you can change the code further to invoke kvm_lapic_reset() after static_branch_inc(&apic_hw_disabled.key)? It's just a bit messy that you have a call back to kvm_lapic_set_base() in there, so perhaps something like this can help:
I'd rather not touch kvm_lapic_reset(). KVM doesn't emulate RESET, and I don't want to make assumptions about why userspace is forcing x2APIC => xAPIC. If userspace wants to propery emulate RESET, it can use KVM_SET_LAPIC.
My preference is to do a light tweak on the original patch, with a rewritten shortlog and changelog. And because I spent way, way too much time digging into this, I went a bit overboard...
From: Sean Christopherson seanjc@google.com Date: Tue, 10 Jan 2023 10:40:33 -0800 Subject: [PATCH] KVM: x86: Reinitialize xAPIC ID when userspace forces x2APIC => xAPIC MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit
Reinitialize the xAPIC ID to the vCPU ID when userspace forces the APIC to transition directly from x2APIC to xAPIC mode, e.g. to emulate RESET. KVM already stuffs the xAPIC ID when the APIC is transitioned from DISABLED to xAPIC (commit 49bd29ba1dbd ("KVM: x86: reset APIC ID when enabling LAPIC")), i.e. userspace is conditioned to expect KVM to update the xAPIC ID, but KVM doesn't handle the architecturally-impossible case where userspace forces x2APIC=>xAPIC via KVM_SET_MSRS.
On its own, the "bug" is benign, as userspace emulation of RESET will also stuff APIC registers via KVM_SET_LAPIC, i.e. will manually set the xAPIC ID. However, commit 3743c2f02517 ("KVM: x86: inhibit APICv/AVIC on changes to APIC ID or APIC base") introduced a bug, fixed by commit commit ef40757743b4 ("KVM: x86: fix APICv/x2AVIC disabled when vm reboot by itself"), that caused KVM to fail to properly update the xAPIC ID when handling KVM_SET_LAPIC. Refresh the xAPIC ID even though it's not strictly necessary so that KVM provides consistent behavior.
Note, KVM follows Intel architecture with regard to handling the xAPIC ID and x2APIC IDs across mode transitions. For the APIC DISABLED case (commit 49bd29ba1dbd), Intel's SDM says the xAPIC ID _may_ be reinitialized
10.4.3 Enabling or Disabling the Local APIC
When IA32_APIC_BASE[11] is set to 0, prior initialization to the APIC may be lost and the APIC may return to the state described in Section 10.4.7.1, “Local APIC State After Power-Up or Reset.”
10.4.7.1 Local APIC State After Power-Up or Reset
... The local APIC ID register is set to a unique APIC ID. ...
i.e. KVM's behavior is legal as per Intel's architecture. In practice, Intel's behavior is N/A as modern Intel CPUs (since at least Haswell) make the xAPIC ID fully read-only.
And for xAPIC => x2APIC transitions (commit 257b9a5faab5 ("KVM: x86: use correct APIC ID on x2APIC transition")), Intel's SDM says:
Any APIC ID value written to the memory-mapped local APIC ID register is not preserved.
AMD's APM says nothing (that I could find) about the xAPIC ID when the APIC is DISABLED, but testing on bare metal (Rome) shows that the xAPIC ID is preserved when the APIC is DISABLED and re-enabled in xAPIC mode. AMD also preserves the xAPIC ID when the APIC is transitioned from xAPIC to x2APIC, i.e. allows a backdoor write of the x2APIC ID, which is again not emulated by KVM.
Reported-by: Emanuele Giuseppe Esposito eesposit@redhat.com Signed-off-by: Sean Christopherson seanjc@google.com --- arch/x86/kvm/lapic.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-)
diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c index 80f92cbc4029..79141d76ad49 100644 --- a/arch/x86/kvm/lapic.c +++ b/arch/x86/kvm/lapic.c @@ -2485,8 +2485,12 @@ void kvm_lapic_set_base(struct kvm_vcpu *vcpu, u64 value) } }
- if (((old_value ^ value) & X2APIC_ENABLE) && (value & X2APIC_ENABLE)) - kvm_apic_set_x2apic_id(apic, vcpu->vcpu_id); + if ((old_value ^ value) & X2APIC_ENABLE) { + if (value & X2APIC_ENABLE) + kvm_apic_set_x2apic_id(apic, vcpu->vcpu_id); + else if (value & MSR_IA32_APICBASE_ENABLE) + kvm_apic_set_xapic_id(apic, vcpu->vcpu_id); + }
if ((old_value ^ value) & (MSR_IA32_APICBASE_ENABLE | X2APIC_ENABLE)) { kvm_make_request(KVM_REQ_APICV_UPDATE, vcpu);
base-commit: 91dc252b0dbb6879e4067f614df1e397fec532a1