From adbfb12d4c4517a8adde23a7fc46538953d56eea Mon Sep 17 00:00:00 2001 From: Hou Wenlong Date: Tue, 2 Nov 2021 17:15:32 +0800 Subject: [PATCH] KVM: x86: Exit to userspace if emulation prepared a completion callback em_rdmsr() and em_wrmsr() return X86EMUL_IO_NEEDED if MSR accesses required an exit to userspace. However, x86_emulate_insn() doesn't return X86EMUL_*, so x86_emulate_instruction() doesn't directly act on X86EMUL_IO_NEEDED; instead, it looks for other signals to differentiate between PIO, MMIO, etc. causing RDMSR/WRMSR emulation not to exit to userspace now. Nevertheless, if the userspace_msr_exit_test testcase in selftests is changed to test RDMSR/WRMSR with a forced emulation prefix, the test passes. What happens is that first userspace exit information is filled but the userspace exit does not happen. Because x86_emulate_instruction() returns 1, the guest retries the instruction---but this time RIP has already been adjusted past the forced emulation prefix, so the guest executes RDMSR/WRMSR and the userspace exit finally happens. Since the X86EMUL_IO_NEEDED path has provided a complete_userspace_io callback, x86_emulate_instruction() can just return 0 if the callback is not NULL. Then RDMSR/WRMSR instruction emulation will exit to userspace directly, without the RDMSR/WRMSR vmexit. Fixes: 1ae099540e8c7 ("KVM: x86: Allow deflecting unknown MSR accesses to user space") Signed-off-by: Hou Wenlong Signed-off-by: Paolo Bonzini Message-Id: <56f9df2ee5c05a81155e2be366c9dc1f7adc8817.1635842679.git.houwenlong93@linux.alibaba.com> --- arch/x86/kvm/x86.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 16f7d20ed19c..1aaf37e1bd0f 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -8227,6 +8227,9 @@ restart: writeback = false; r = 0; vcpu->arch.complete_userspace_io = complete_emulated_mmio; + } else if (vcpu->arch.complete_userspace_io) { + writeback = false; + r = 0; } else if (r == EMULATION_RESTART) goto restart; else