On Mon, Jul 23, 2018 at 12:33 PM Sean Christopherson
<sean.j.christopherson@xxxxxxxxx> wrote:
>
> 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 44ea2b1758d8 ("KVM: VMX: Move MSR_KERNEL_GS_BASE out of the vmx
> autoload msr area")
> [2] commit 5897297bc228 ("KVM: VMX: Don't intercept MSR_KERNEL_GS_BASE")
> [3] commit c8770e7ba63b ("KVM: VMX: Fix host userspace gsbase corruption")
>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
Reviewed-by: Peter Shier <pshier@xxxxxxxxxx>
Tested-by: Peter Shier <pshier@xxxxxxxxxx>
