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Subject[PATCH 5.15 060/262] KVM: x86: Exit to userspace if emulation prepared a completion callback
Date
From: Hou Wenlong <houwenlong93@linux.alibaba.com>

[ Upstream commit adbfb12d4c4517a8adde23a7fc46538953d56eea ]

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 <houwenlong93@linux.alibaba.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <56f9df2ee5c05a81155e2be366c9dc1f7adc8817.1635842679.git.houwenlong93@linux.alibaba.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
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 6b76486702ded..8213f7fb71a7b 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8068,6 +8068,9 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
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
--
2.34.1


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