Re: [DROP PATCH v2] KVM: x86/pmu: Make top-down.slots event unavailable in supported leaf
From: Like Xu
Date: Thu Jan 06 2022 - 23:13:44 EST
On 7/1/2022 4:12 am, Jim Mattson wrote:
On Thu, Jan 6, 2022 at 10:09 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
On Thu, Jan 06, 2022, Like Xu wrote:
From: Like Xu <likexu@xxxxxxxxxxx>
When we choose to disable the fourth fixed counter TOPDOWN.SLOTS,
we also need to comply with the specification and set 0AH.EBX.[bit 7]
to 1 if the guest (e.g. on the ICX) has a value of 0AH.EAX[31:24] > 7.
Fixes: 2e8cd7a3b8287 ("kvm: x86: limit the maximum number of vPMU fixed counters to 3")
Signed-off-by: Like Xu <likexu@xxxxxxxxxxx>
---
v1 -> v2 Changelog:
- Make it simpler to keep forward compatibility; (Sean)
- Wrap related comment at ~80 chars; (Sean)
Previous:
https://lore.kernel.org/kvm/20220105050711.67280-1-likexu@xxxxxxxxxxx/
arch/x86/kvm/cpuid.c | 12 ++++++++++++
1 file changed, 12 insertions(+)
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 0b920e12bb6d..4fe17a537084 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -782,6 +782,18 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
eax.split.mask_length = cap.events_mask_len;
edx.split.num_counters_fixed = min(cap.num_counters_fixed, MAX_FIXED_COUNTERS);
+
+ /*
+ * The 8th Intel architectural event (Topdown Slots) will be supported
Nit, the "8th" part is unnecessary information.
+ * if the 4th fixed counter exists && EAX[31:24] > 7 && EBX[7] = 0.
+ *
+ * Currently, KVM needs to set EAX[31:24] < 8 or EBX[7] == 1
+ * to make this event unavailable in a consistent way.
+ */
This comment is now slightly stale. It also doesn't say why the event is made
unavailable.
+ if (edx.split.num_counters_fixed < 4 &&
Rereading the changelog and the changelog of the Fixed commit, I don't think KVM
should bother checking num_counters_fixed. IIUC, cap.events_mask[7] should already
be '1' if there are less than 4 fixed counters in hardware, but at the same time
there's no harm in being a bit overzealous. That would help simplifiy the comment
as there's no need to explain why num_counters_fixed is checked, e.g. the fact that
Topdown Slots uses the 4th fixed counter is irrelevant with respect to the legality
of setting EBX[7]=1 to hide an unsupported event.
I was under the impression that the CPUID.0AH:EBX bits were
independent of the fixed counters. That is, if CPUID.0AH:EAX[31:24] >
7 and CPUID.0AH:EBX[7] is clear, then one should be able to program a
general purpose counter with event selector A4H and umask 01H,
regardless of whether or not fixed counter 4 exists.
The impression stepped on my toes. This patch is pointless and magnifies my
clumsiness.
Now the self-contradiction comes to the CLX and older platforms,
0xA. EAX. Bits 31 - 24: Length of EBX bit vector to enumerate architectural
performance monitoring events. Architectural event x is supported
if EBX[x]=0 && EAX[31:24]>x.
however actually for CLX, its CPUID.0xA.EBX[7] = 0 && EAX[31:24] = 0x7 and
we can do exactly the following on a CLX without fixed ctr3
$ perf stat -e cpu/event=0xa4,umask=0x01/ ls
Performance counter stats for 'ls':
2,448,185 cpu/event=0xa4,umask=0x01/
So in the context of Intel, we have a performance event that can share the
architectural encoding, but it is not an architectural-defined event.
This is strange, did I miss something somewhere ?
/*
* Hide Intel's Topdown Slots architectural event, it's not yet
* supported by KVM.
*/
if (eax.split.mask_length > 7)
cap.events_mask |= BIT_ULL(7);
+ eax.split.mask_length > 7)
+ cap.events_mask |= BIT_ULL(7);
+
edx.split.bit_width_fixed = cap.bit_width_fixed;
if (cap.version)
edx.split.anythread_deprecated = 1;
--
2.33.1