Re: [Xen-devel] [v3 04/12] x86/fsgsbase/64: Enable FSGSBASE instructions in the helper functions

[Juergen Gross]

On 24/10/2018 21:41, Andrew Cooper wrote:
> On 24/10/18 20:16, Andy Lutomirski wrote:
>> On Tue, Oct 23, 2018 at 11:43 AM Chang S. Bae <chang.seok.bae@xxxxxxxxx> wrote:
>>> The helper functions will switch on faster accesses to FSBASE and GSBASE
>>> when the FSGSBASE feature is enabled.
>>>
>>> Accessing user GSBASE needs a couple of SWAPGS operations. It is avoidable
>>> if the user GSBASE is saved at kernel entry, being updated as changes, and
>>> restored back at kernel exit. However, it seems to spend more cycles for
>>> savings and restorations. Little or no benefit was measured from
>>> experiments.
>>>
>>> Signed-off-by: Chang S. Bae <chang.seok.bae@xxxxxxxxx>
>>> Reviewed-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
>>> Cc: Any Lutomirski <luto@xxxxxxxxxx>
>>> Cc: H. Peter Anvin <hpa@xxxxxxxxx>
>>> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>>> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
>>> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
>>> ---
>>>  arch/x86/include/asm/fsgsbase.h | 17 +++----
>>>  arch/x86/kernel/process_64.c    | 82 +++++++++++++++++++++++++++------
>>>  2 files changed, 75 insertions(+), 24 deletions(-)
>>>
>>> diff --git a/arch/x86/include/asm/fsgsbase.h b/arch/x86/include/asm/fsgsbase.h
>>> index b4d4509b786c..e500d771155f 100644
>>> --- a/arch/x86/include/asm/fsgsbase.h
>>> +++ b/arch/x86/include/asm/fsgsbase.h
>>> @@ -57,26 +57,23 @@ static __always_inline void wrgsbase(unsigned long gsbase)
>>>                         : "memory");
>>>  }
>>>
>>> +#include <asm/cpufeature.h>
>>> +
>>>  /* Helper functions for reading/writing FS/GS base */
>>>
>>>  static inline unsigned long x86_fsbase_read_cpu(void)
>>>  {
>>>         unsigned long fsbase;
>>>
>>> -       rdmsrl(MSR_FS_BASE, fsbase);
>>> +       if (static_cpu_has(X86_FEATURE_FSGSBASE))
>>> +               fsbase = rdfsbase();
>>> +       else
>>> +               rdmsrl(MSR_FS_BASE, fsbase);
>>>
>>>         return fsbase;
>>>  }
>>>
>>> -static inline unsigned long x86_gsbase_read_cpu_inactive(void)
>>> -{
>>> -       unsigned long gsbase;
>>> -
>>> -       rdmsrl(MSR_KERNEL_GS_BASE, gsbase);
>>> -
>>> -       return gsbase;
>>> -}
>>> -
>>> +extern unsigned long x86_gsbase_read_cpu_inactive(void);
>>>  extern void x86_fsbase_write_cpu(unsigned long fsbase);
>>>  extern void x86_gsbase_write_cpu_inactive(unsigned long gsbase);
>>>
>>> diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
>>> index 31b4755369f0..fcf18046c3d6 100644
>>> --- a/arch/x86/kernel/process_64.c
>>> +++ b/arch/x86/kernel/process_64.c
>>> @@ -159,6 +159,36 @@ enum which_selector {
>>>         GS
>>>  };
>>>
>>> +/*
>>> + * Interrupts are disabled here. Out of line to be protected from kprobes.
>>> + */
>>> +static noinline __kprobes unsigned long rd_inactive_gsbase(void)
>>> +{
>>> +       unsigned long gsbase, flags;
>>> +
>>> +       local_irq_save(flags);
>>> +       native_swapgs();
>>> +       gsbase = rdgsbase();
>>> +       native_swapgs();
>>> +       local_irq_restore(flags);
>>> +
>>> +       return gsbase;
>>> +}
>> Please fold this into its only caller and make *that* noinline.
>>
>> Also, this function, and its "write" equivalent, will access the
>> *active* gsbase.  So it either needs to be fixed for Xen PV or some
>> clear comment and careful auditing needs to be added to ensure that
>> it's not used on Xen PV.  Or it needs to be renamed
>> native_x86_fsgsbase_... and add paravirt hooks, since Xen PV allows a
>> very efficient but different implementation, I think.  The latter is
>> probably the right solution.
>>
>> (Hi Xen people -- how does CR4.FSGSBASE work on Xen?  Is it always
>> set?  Never set?  Set only if the guest tries to set it?)
> 
> FML. Seriously - whoever put this code into the hypervisor in the past
> did an atrocious job. After some experimentation, you're going to be
> sad and I'm declaring this borderline unusable.
> 
> Looks like Xen unconditionally enabled CR4.FSGSBASE if it is available.Â
> Therefore, PV guests can use the instructions, even if the bit is clear
> in vCR4.
> 
> The CPUID bits are exposed to guests by default, and Xen will emulate
> vCR4.FSGSBASE being set and cleared.
> 
> We don't however emulate swapgs (which is a cpl0 instruction). The
> guest gets handed a #GP[0] instead.
> 
> The Linux WRMSR PVop uses the set_segment_base() hypercall in instead of
> going through the full wrmsr emulation path.
> 
> There is no equivalent get hypercall, so the only way I can see of
> getting the value is to actually read MSR_KERNEL_GS_BASE and take the
> full rdmsr emulation path.

Or shadow the value in a percpu variable.


Juergen