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SubjectRe: [PATCH v2] KVM: x86: do not read FS/GS base MSRs when saving them
> On Jan 2, 2018, at 5:59 AM, Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> The FS and userspace GS bases are available in current->thread, while the
> kernel GS base is a percpu variable. Skip the expensive rdmsr and just
> get the values from memory.

That fsbase change is wrong: thread->fsbase is not guaranteed to be
correct for current.

>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
> v1->v2: hide the accessor for 32-bit kernels
>
> arch/x86/include/asm/desc.h | 8 ++++++++
> arch/x86/include/asm/kvm_host.h | 10 ----------
> arch/x86/kernel/cpu/common.c | 1 +
> arch/x86/kvm/svm.c | 2 +-
> arch/x86/kvm/vmx.c | 17 +++--------------
> 5 files changed, 13 insertions(+), 25 deletions(-)
>
> diff --git a/arch/x86/include/asm/desc.h b/arch/x86/include/asm/desc.h
> index 4011cb03ef08..6ef8c47d0baa 100644
> --- a/arch/x86/include/asm/desc.h
> +++ b/arch/x86/include/asm/desc.h
> @@ -85,6 +85,14 @@ static inline phys_addr_t get_cpu_gdt_paddr(unsigned int cpu)
> return per_cpu_ptr_to_phys(get_cpu_gdt_rw(cpu));
> }
>
> +#ifdef CONFIG_X86_64
> +/* Provide the current kernel GS base. */
> +static inline void *get_current_kernel_gs_base(void)
> +{
> + return this_cpu_ptr(irq_stack_union.gs_base);
> +}
> +#endif

This is an awful name because MSR_KERNEL_GS_BASE means the user gs
base. How about calling it something like
get_this_cpu_kernelmode_gs_base() or similar?

> #ifdef CONFIG_X86_64
> - vmcs_writel(HOST_FS_BASE, read_msr(MSR_FS_BASE));
> - vmcs_writel(HOST_GS_BASE, read_msr(MSR_GS_BASE));
> + vmcs_writel(HOST_FS_BASE, current->thread.fsbase);

That's wrong. thread->fsbase isn't kept up to date while the thread
is running. You could potentially try to expose an interface to get
save_base_legacy() called to update it.

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