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Sean Christopherson authored
When lazy save/restore of MSR_KERNEL_GS_BASE was introduced[1], the MSR was intercepted in all modes and was only restored for the host when the guest is in 64-bit mode. So at the time, going through the full host restore prior to accessing MSR_KERNEL_GS_BASE was necessary to load host state and was not a significant waste of cycles. Later, MSR_KERNEL_GS_BASE interception was disabled for a 64-bit guest[2], and then unconditionally saved/restored for the host[3]. As a result, loading full host state is overkill for accesses to MSR_KERNEL_GS_BASE, and completely unnecessary when the guest is not in 64-bit mode. Add a dedicated utility to read/write the guest's MSR_KERNEL_GS_BASE (outside of the save/restore flow) to minimize the overhead incurred when accessing the MSR. When setting EFER, only decache the MSR if the new EFER will disable long mode. Removing out-of-band usage of vmx_load_host_state() also eliminates, or at least reduces, potential corner cases in its usage, which in turn will (hopefuly) make it easier to reason about future changes to the save/restore flow, e.g. optimization of saving host state. [1] commit 44ea2b17 ("KVM: VMX: Move MSR_KERNEL_GS_BASE out of the vmx autoload msr area") [2] commit 5897297b ("KVM: VMX: Don't intercept MSR_KERNEL_GS_BASE") [3] commit c8770e7b ("KVM: VMX: Fix host userspace gsbase corruption") Signed-off-by:
Sean Christopherson <sean.j.christopherson@intel.com> Signed-off-by:
Paolo Bonzini <pbonzini@redhat.com>
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