Re: [PATCH v2 01/10] KVM: x86: Enforce x2APIC's must-be-zero reserved ICR bits

[Sean Christopherson <seanjc@google.com>]

On Fri, Jul 19, 2024, Sean Christopherson wrote:
> Inject a #GP on a WRMSR(ICR) that attempts to set any reserved bits that
> are must-be-zero on both Intel and AMD, i.e. any reserved bits other than
> the BUSY bit, which Intel ignores and basically says is undefined.
> 
> KVM's xapic_state_test selftest has been fudging the bug since commit
> 4b88b1a518b3 ("KVM: selftests: Enhance handling WRMSR ICR register in
> x2APIC mode"), which essentially removed the testcase instead of fixing
> the bug.
> 
> WARN if the nodecode path triggers a #GP, as the CPU is supposed to check
> reserved bits for ICR when it's partially virtualized.

Apparently this isn't accurate, as I've now hit the WARN twice with x2AVIC.  I
haven't debugged in depth, but it's either INVALID_TARGET and INVALID_INT_TYPE.
Which is odd, because the WARN only happens rarely, e.g. appears to be a race of
some form.  But I wouldn't expect those checks to be subject to races.

Ah, but maybe this one is referring to the VALID bit?

  address is not present in the physical or logical ID tables

If that's the case, then (a) ucode is buggy (IMO) and is doing table lookups
*before* reserved bits checks, and (b) I don't see a better option than simply
deleting the WARN.

  ------------[ cut here ]------------
  WARNING: CPU: 146 PID: 274555 at arch/x86/kvm/lapic.c:2521 kvm_apic_write_nodecode+0x7a/0x90 [kvm]
  Modules linked in: kvm_amd kvm ... [last unloaded: kvm]
  CPU: 146 UID: 0 PID: 274555 Comm: qemu Not tainted 6.12.0-smp--41585e8a34cb-sink #458
  Hardware name: Google Astoria/astoria, BIOS 0.20240617.0-0 06/17/2024
  RIP: 0010:kvm_apic_write_nodecode+0x7a/0x90 [kvm]
  RSP: 0018:ff51c04b4d133be8 EFLAGS: 00010202
  RAX: 0000000000000001 RBX: 0000000000000000 RCX: 00000000000cffff
  RDX: 0000000087fd0e00 RSI: 00000000000cffff RDI: ff42132c9e336f00
  RBP: ff51c04b4d133e50 R08: 0000000000000000 R09: 0000000000060000
  R10: ffffffffc067428f R11: ffffffffc080aa20 R12: 00000000000cffff
  R13: 0000000000000000 R14: ff42132d09e7c2c0 R15: 0000000000000000
  FS:  00007fc1af0006c0(0000) GS:ff42138a08500000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 0000000000000000 CR3: 0000006267e52001 CR4: 0000000000771ef0
  PKRU: 00000000
  Call Trace:
   <TASK>
   avic_incomplete_ipi_interception+0x24a/0x4c0 [kvm_amd]
   kvm_arch_vcpu_ioctl_run+0x1e11/0x2720 [kvm]
   kvm_vcpu_ioctl+0x54f/0x630 [kvm]
   __se_sys_ioctl+0x6b/0xc0
   do_syscall_64+0x83/0x160
   entry_SYSCALL_64_after_hwframe+0x76/0x7e
  RIP: 0033:0x7fc1b584624b
   </TASK>
  ---[ end trace 0000000000000000 ]---