linux-kernel - Re: [PATCH 12/22] x86/fpu: Only write PKRU if it is different from current

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Date:   Fri, 8 Mar 2019 18:24:34 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        kvm@...r.kernel.org, "Jason A. Donenfeld" <Jason@...c4.com>,
        Rik van Riel <riel@...riel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH 12/22] x86/fpu: Only write PKRU if it is different from
 current

On 2019-02-25 10:08:10 [-0800], Dave Hansen wrote:
> On 2/21/19 3:50 AM, Sebastian Andrzej Siewior wrote:
> > @@ -111,6 +111,12 @@ static inline void __write_pkru(u32 pkru)
> >  {
> >  	u32 ecx = 0, edx = 0;
> >  
> > +	/*
> > +	 * WRPKRU is relatively expensive compared to RDPKRU.
> > +	 * Avoid WRPKRU when it would not change the value.
> > +	 */
> > +	if (pkru == __read_pkru())
> > +		return;
> >  	/*
> >  	 * "wrpkru" instruction.  Loads contents in EAX to PKRU,
> >  	 * requires that ecx = edx = 0.
> 
> Could we do this in write_pkru() instead?  I tried to keep the __
> versions of read and write as simple and symmetric as possible.

write_pkru() has:
| static inline void write_pkru(u32 pkru)
| {
|         struct pkru_state *pk;
| 
|         if (!boot_cpu_has(X86_FEATURE_OSPKE))
|                 return;
|         
|         pk = get_xsave_addr(&current->thread.fpu.state.xsave, XFEATURE_PKRU);
|         /*
|          * The PKRU value in xstate needs to be in sync with the value that is
|          * written to the CPU. The FPU restore on return to userland would
|          * otherwise load the previous value again.
|          */
|         fpregs_lock();
|         if (pk)
|                 pk->pkru = pkru;
|         __write_pkru(pkru);
|         fpregs_unlock();
| }

and is other callers besides write_pkru():
|arch/x86/include/asm/fpu/internal.h:    __write_pkru(pkru_val);
|arch/x86/kvm/vmx/vmx.c:         __write_pkru(vcpu->arch.pkru);
|arch/x86/kvm/vmx/vmx.c:                 __write_pkru(vmx->host_pkru)

Do want __write_pkru_isn() and __read_pkru_isn() so those are symmetric
and then we would have __write_pkru() which does the read check and
invokes __write_pkru_isn()?

Sebastian