linux-kernel - Re: [PATCH v2] KVM: x86: do not read FS/GS base MSRs when saving them

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Date:   Wed, 3 Jan 2018 14:20:38 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>,
        Andrew Lutomirski <luto@...nel.org>
Subject: Re: [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@...hat.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@...hat.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.