Re: [PATCH 02/15] KVM: x86/xen: fix Xen hypercall page msr handling

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On 04.12.20 02:18, David Woodhouse wrote:
From: Joao Martins <joao.m.martins@xxxxxxxxxx>

Xen usually places its MSR at 0x40000000 or 0x40000200 depending on
whether it is running in viridian mode or not. Note that this is not
ABI guaranteed, so it is possible for Xen to advertise the MSR some
place else.

Given the way xen_hvm_config() is handled, if the former address is
selected, this will conflict with Hyper-V's MSR
(HV_X64_MSR_GUEST_OS_ID) which unconditionally uses the same address.

Given that the MSR location is arbitrary, move the xen_hvm_config()
handling to the top of kvm_set_msr_common() before falling through.

Signed-off-by: Joao Martins <joao.m.martins@xxxxxxxxxx>
Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
---
  arch/x86/kvm/x86.c | 5 +++--
  1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index c7f1ba21212e..13ba4a64f748 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3001,6 +3001,9 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
  	u32 msr = msr_info->index;
  	u64 data = msr_info->data;
+ if (msr && (msr == vcpu->kvm->arch.xen_hvm_config.msr))
+		return xen_hvm_config(vcpu, data);

How much of this can we handle with the MSR allow listing and MSR trapping in user space?


Alex



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