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SubjectRe: [Xen-devel] [v3 04/12] x86/fsgsbase/64: Enable FSGSBASE instructions in the helper functions
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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@intel.com> 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@intel.com>
>>> Reviewed-by: Andi Kleen <ak@linux.intel.com>
>>> Cc: Any Lutomirski <luto@kernel.org>
>>> Cc: H. Peter Anvin <hpa@zytor.com>
>>> Cc: Thomas Gleixner <tglx@linutronix.de>
>>> Cc: Ingo Molnar <mingo@kernel.org>
>>> Cc: Dave Hansen <dave.hansen@linux.intel.com>
>>> ---
>>> 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. 
> 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

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