Re: [PATCH v5 13/19] x86/cpufeatures: Add flag to track whether MSR IA32_FEAT_CTL is configured

Jacob Keller
2020-02-25T21:49:22Z

Hi Sean,

> Add a new feature flag, X86_FEATURE_MSR_IA32_FEAT_CTL, to track whether
> IA32_FEAT_CTL has been initialized. This will allow KVM, and any future
> subsystems that depend on IA32_FEAT_CTL, to rely purely on cpufeatures
> to query platform support, e.g. allows a future patch to remove KVM's
> manual IA32_FEAT_CTL MSR checks.
>
> Various features (on platforms that support IA32_FEAT_CTL) are dependent
> on IA32_FEAT_CTL being configured and locked, e.g. VMX and LMCE. The
> MSR is always configured during boot, but only if the CPU vendor is
> recognized by the kernel. Because CPUID doesn't incorporate the current
> IA32_FEAT_CTL value in its reporting of relevant features, it's possible
> for a feature to be reported as supported in cpufeatures but not truly
> enabled, e.g. if the CPU supports VMX but the kernel doesn't recognize
> the CPU.
>
> As a result, without the flag, KVM would see VMX as supported even if
> IA32_FEAT_CTL hasn't been initialized, and so would need to manually
> read the MSR and check the various enabling bits to avoid taking an
> unexpected #GP on VMXON.

I recently ran into a general protection fault that I believe is the
fault of this patch:

> [ 32.189584] general protection fault, maybe for address 0xffffb567801bcf58: 0000 [#1] SMP PTI
> [ 32.198103] CPU: 1 PID: 2600 Comm: rngd Not tainted 5.6.0-rc2-jk+ #2
> [ 32.204454] Hardware name: Intel Corporation S2600STQ/S2600STQ, BIOS SE5C620.86B.02.01.0008.031920191559 03/19/2019
> [ 32.214887] RIP: 0010:hardware_enable+0x100/0x1a0 [kvm_intel]
> [ 32.220628] Code: 00 00 48 39 f8 74 0f 65 48 89 3d 43 a2 cb 3c e8 66 d3 cc c5 66 90 48 89 df 57 9d 0f 1f 44 00 00 bf 01 00 00 00 e8 90 3d ca c5 <f3> 0f c7 34 24 31 c0 80 3d 59 8d 03 00 00 75 36 48 8b 5c 24 10 65
> [ 32.239373] RSP: 0000:ffffb567801bcf58 EFLAGS: 00010002
> [ 32.244598] RAX: 0000000000300000 RBX: 0000000000000086 RCX: ffff8f2650440000
> [ 32.251730] RDX: 0000000000300000 RSI: 0000000000000000 RDI: ffff8f2650457020
> [ 32.258862] RBP: 0000000000000007 R08: 000000077ea5d531 R09: 0000000000000000
> [ 32.265986] R10: 000001432bf20982 R11: 0000000000000000 R12: ffffd55b80467110
> [ 32.273118] R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
> [ 32.280243] FS: 00007facfe66f700(0000) GS:ffff8f2650440000(0000) knlGS:0000000000000000
> [ 32.288329] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [ 32.294077] CR2: 00007facf0003000 CR3: 0000000b7d402006 CR4: 00000000007626e0
> [ 32.301210] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> [ 32.308342] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> [ 32.315474] PKRU: 55555554
> [ 32.318186] Call Trace:
> [ 32.320642] <IRQ>
> [ 32.322689] kvm_arch_hardware_enable+0x84/0x240 [kvm]
> [ 32.327836] hardware_enable_nolock+0x31/0x60 [kvm]
> [ 32.332717] flush_smp_call_function_queue+0x4d/0xe0
> [ 32.337683] smp_call_function_interrupt+0x3a/0xd0
> [ 32.342471] call_function_interrupt+0xf/0x20
> [ 32.346830] </IRQ>
> [ 32.348935] RIP: 0033:0x7facffd4c753
> [ 32.352514] Code: e8 48 c7 45 e0 00 00 00 00 eb 5f 48 8b 45 c8 48 8b 50 38 48 8b 45 c8 8b 40 40 89 c0 48 01 d0 48 89 45 f0 48 8b 45 f0 0f b6 00 <83> c0 01 89 c2 48 8b 45 f0 88 10 48 8b 45 c8 8b 50 40 48 8b 45 c8
> [ 32.371263] RSP: 002b:00007facfe66ebf0 EFLAGS: 00000202 ORIG_RAX: ffffffffffffff03
> [ 32.378826] RAX: 00000000000000ee RBX: 0000000000004097 RCX: 0000000000000000
> [ 32.385961] RDX: 0000562781dbadf0 RSI: 0000000000000000 RDI: 00007ffd7edf9080
> [ 32.393092] RBP: 00007facfe66ec30 R08: 00007ffd7edf9080 R09: 000000000000cd4a
> [ 32.400226] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
> [ 32.407358] R13: 00007facf0000b20 R14: 0000562781dba2e8 R15: 00007facfe66ed10
> [ 32.414493] Modules linked in: ip6table_mangle ip6table_nat iptable_mangle iptable_nat nf_nat nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 libcrc32c ebtable_filter ebtables ip6table_filter ip6_tables iptable_filter rfkill ib_isert iscsi
> _target_mod ib_srpt target_core_mod ib_srp scsi_transport_srp ib_ipoib vfat fat ib_umad rpcrdma sunrpc intel_rapl_msr intel_rapl_common rdma_ucm ib_iser rdma_cm isst_if_common iw_cm ib_cm libiscsi skx_edac scsi_transport_iscsi nfit libnv
> dimm x86_pkg_temp_thermal intel_powerclamp coretemp kvm_intel kvm irqbypass crct10dif_pclmul crc32_pclmul ghash_clmulni_intel intel_cstate i40iw qat_c62x iTCO_wdt ipmi_ssif iTCO_vendor_support ib_uverbs mei_me intel_qat intel_uncore ib_c
> ore joydev intel_rapl_perf pcspkr ipmi_si authenc ioatdma mei i2c_i801 lpc_ich dca ipmi_devintf ipmi_msghandler acpi_power_meter acpi_pad ip_tables ast i2c_algo_bit drm_vram_helper drm_ttm_helper ttm drm_kms_helper cec drm ice i40e crc32
> c_intel wmi fuse
> [ 32.498314] ---[ end trace bfeeeba337a01208 ]---

I noticed that a slightly older commit from before this does not fail.
Additionally, the system reports the following during boot:

kvm: disabled by bios

I looked into the vmx_disabled_by_bios and noticed that it checks for
both X86_FEATURE_MSR_IA32_FEAT_CTL and X86_FEATURE_VMX.

Compared to the older code before commit a4d0b2fdbcf7 ("KVM: VMX: Use
VMX feature flag to query BIOS enabling") it's not clear to me how
exactly this could fail to match up.

I suspect something is wrong and the features are enabled even though
the BIOS has it disabled, leading to later failure because of this.

Thanks,
Jake