linux-kernel - Re: [PATCHv6 04/11] x86/mm: Handle LAM on context switch

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Date:   Mon, 15 Aug 2022 20:02:17 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>, x86@...nel.org,
        Kostya Serebryany <kcc@...gle.com>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Alexander Potapenko <glider@...gle.com>,
        Taras Madan <tarasmadan@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        "H . J . Lu" <hjl.tools@...il.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCHv6 04/11] x86/mm: Handle LAM on context switch

On Mon, Aug 15, 2022 at 08:37:25PM +0300, Kirill A. Shutemov wrote:
> On Mon, Aug 15, 2022 at 03:42:25PM +0200, Peter Zijlstra wrote:
> > On Mon, Aug 15, 2022 at 07:17:56AM +0300, Kirill A. Shutemov wrote:
> > > diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
> > > index c1e31e9a85d7..fdc0b69b5da7 100644
> > > --- a/arch/x86/mm/tlb.c
> > > +++ b/arch/x86/mm/tlb.c
> > > @@ -154,17 +154,18 @@ static inline u16 user_pcid(u16 asid)
> > >  	return ret;
> > >  }
> > >  
> > > -static inline unsigned long build_cr3(pgd_t *pgd, u16 asid)
> > > +static inline unsigned long build_cr3(pgd_t *pgd, u16 asid, unsigned long lam)
> > >  {
> > >  	if (static_cpu_has(X86_FEATURE_PCID)) {
> > > -		return __sme_pa(pgd) | kern_pcid(asid);
> > > +		return __sme_pa(pgd) | kern_pcid(asid) | lam;
> > >  	} else {
> > >  		VM_WARN_ON_ONCE(asid != 0);
> > > -		return __sme_pa(pgd);
> > > +		return __sme_pa(pgd) | lam;
> > >  	}
> > >  }
> > >  
> > > -static inline unsigned long build_cr3_noflush(pgd_t *pgd, u16 asid)
> > > +static inline unsigned long build_cr3_noflush(pgd_t *pgd, u16 asid,
> > > +					      unsigned long lam)
> > >  {
> > >  	VM_WARN_ON_ONCE(asid > MAX_ASID_AVAILABLE);
> > >  	/*
> > > @@ -173,7 +174,7 @@ static inline unsigned long build_cr3_noflush(pgd_t *pgd, u16 asid)
> > >  	 * boot because all CPU's the have same capabilities:
> > >  	 */
> > >  	VM_WARN_ON_ONCE(!boot_cpu_has(X86_FEATURE_PCID));
> > > -	return __sme_pa(pgd) | kern_pcid(asid) | CR3_NOFLUSH;
> > > +	return __sme_pa(pgd) | kern_pcid(asid) | lam | CR3_NOFLUSH;
> > >  }
> > 
> > Looking at this; I wonder if we want something like this:
> > 
> > --- a/arch/x86/mm/tlb.c
> > +++ b/arch/x86/mm/tlb.c
> > @@ -157,6 +157,7 @@ static inline u16 user_pcid(u16 asid)
> >  static inline unsigned long build_cr3(pgd_t *pgd, u16 asid, unsigned long lam)
> >  {
> >  	if (static_cpu_has(X86_FEATURE_PCID)) {
> > +		VM_WARN_ON_ONCE(asid > MAX_ASID_AVAILABLE);
> >  		return __sme_pa(pgd) | kern_pcid(asid) | lam;
> >  	} else {
> >  		VM_WARN_ON_ONCE(asid != 0);
> > @@ -167,14 +168,13 @@ static inline unsigned long build_cr3(pg
> >  static inline unsigned long build_cr3_noflush(pgd_t *pgd, u16 asid,
> >  					      unsigned long lam)
> >  {
> > -	VM_WARN_ON_ONCE(asid > MAX_ASID_AVAILABLE);
> >  	/*
> >  	 * Use boot_cpu_has() instead of this_cpu_has() as this function
> >  	 * might be called during early boot. This should work even after
> >  	 * boot because all CPU's the have same capabilities:
> >  	 */
> >  	VM_WARN_ON_ONCE(!boot_cpu_has(X86_FEATURE_PCID));
> > -	return __sme_pa(pgd) | kern_pcid(asid) | lam | CR3_NOFLUSH;
> > +	return build_cr3(pgd, asid, lam) | CR3_NOFLUSH;
> >  }
> 
> Looks sane, but seems unrelated to the patch. Is it okay to fold it
> anyway?

Related in so far as that it reduces the number of sites where we have
the actual CR3 'computation' (which is how I arrived at the thing).

Arguably we could even do something like:

static inline unsigned long build_cr3(pgd_t *pgd, u16 asid, unsigned long lam)
{
	unsigned long cr3 = __sme_pa(pgd) | lam;

	if (static_cpu_has(X86_FEATURE_PCID)) {
		VM_WARN_ON_ONCE(asid > MAX_ASID_AVAILABLE);
		cr |= kern_pcid(asid);
	} else {
		VM_WARN_ON_ONCE(asid != 0);
	}

	return cr3;
}

But perhaps that's pushing things a little.

IMO fine to fold.