Re: [PATCH 6.6.y] KVM: SVM: Skip fastpath emulation on VM-Exit if next RIP isn't valid

[Sean Christopherson <seanjc@google.com>]

On Tue, Oct 14, 2025, Sasha Levin wrote:
> From: Sean Christopherson <seanjc@google.com>
> 
> [ Upstream commit 0910dd7c9ad45a2605c45fd2bf3d1bcac087687c ]
> 
> Skip the WRMSR and HLT fastpaths in SVM's VM-Exit handler if the next RIP
> isn't valid, e.g. because KVM is running with nrips=false.  SVM must
> decode and emulate to skip the instruction if the CPU doesn't provide the
> next RIP, and getting the instruction bytes to decode requires reading
> guest memory.  Reading guest memory through the emulator can fault, i.e.
> can sleep, which is disallowed since the fastpath handlers run with IRQs
> disabled.
> 
>  BUG: sleeping function called from invalid context at ./include/linux/uaccess.h:106
>  in_atomic(): 1, irqs_disabled(): 1, non_block: 0, pid: 32611, name: qemu
>  preempt_count: 1, expected: 0
>  INFO: lockdep is turned off.
>  irq event stamp: 30580
>  hardirqs last  enabled at (30579): [<ffffffffc08b2527>] vcpu_run+0x1787/0x1db0 [kvm]
>  hardirqs last disabled at (30580): [<ffffffffb4f62e32>] __schedule+0x1e2/0xed0
>  softirqs last  enabled at (30570): [<ffffffffb4247a64>] fpu_swap_kvm_fpstate+0x44/0x210
>  softirqs last disabled at (30568): [<ffffffffb4247a64>] fpu_swap_kvm_fpstate+0x44/0x210
>  CPU: 298 UID: 0 PID: 32611 Comm: qemu Tainted: G     U              6.16.0-smp--e6c618b51cfe-sleep #782 NONE
>  Tainted: [U]=USER
>  Hardware name: Google Astoria-Turin/astoria, BIOS 0.20241223.2-0 01/17/2025
>  Call Trace:
>   <TASK>
>   dump_stack_lvl+0x7d/0xb0
>   __might_resched+0x271/0x290
>   __might_fault+0x28/0x80
>   kvm_vcpu_read_guest_page+0x8d/0xc0 [kvm]
>   kvm_fetch_guest_virt+0x92/0xc0 [kvm]
>   __do_insn_fetch_bytes+0xf3/0x1e0 [kvm]
>   x86_decode_insn+0xd1/0x1010 [kvm]
>   x86_emulate_instruction+0x105/0x810 [kvm]
>   __svm_skip_emulated_instruction+0xc4/0x140 [kvm_amd]
>   handle_fastpath_invd+0xc4/0x1a0 [kvm]
>   vcpu_run+0x11a1/0x1db0 [kvm]
>   kvm_arch_vcpu_ioctl_run+0x5cc/0x730 [kvm]
>   kvm_vcpu_ioctl+0x578/0x6a0 [kvm]
>   __se_sys_ioctl+0x6d/0xb0
>   do_syscall_64+0x8a/0x2c0
>   entry_SYSCALL_64_after_hwframe+0x4b/0x53
>  RIP: 0033:0x7f479d57a94b
>   </TASK>
> 
> Note, this is essentially a reapply of commit 5c30e8101e8d ("KVM: SVM:
> Skip WRMSR fastpath on VM-Exit if next RIP isn't valid"), but with
> different justification (KVM now grabs SRCU when skipping the instruction
> for other reasons).
> 
> Fixes: b439eb8ab578 ("Revert "KVM: SVM: Skip WRMSR fastpath on VM-Exit if next RIP isn't valid"")
> Cc: stable@vger.kernel.org
> Link: https://lore.kernel.org/r/20250805190526.1453366-2-seanjc@google.com
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> [ adapted switch-based MSR/HLT fastpath to if-based MSR-only check ]
> Signed-off-by: Sasha Levin <sashal@kernel.org>
> ---

Acked-by: Sean Christopherson <seanjc@google.com>