linux-kernel - Re: [PATCHv3 4/8] x86/mm: Handle LAM on context switch

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Date:   Wed, 15 Jun 2022 18:54:36 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Cc:     "peterz@...radead.org" <peterz@...radead.org>,
        "Lutomirski, Andy" <luto@...nel.org>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "hjl.tools@...il.com" <hjl.tools@...il.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "kcc@...gle.com" <kcc@...gle.com>,
        "andreyknvl@...il.com" <andreyknvl@...il.com>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "dvyukov@...gle.com" <dvyukov@...gle.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "ryabinin.a.a@...il.com" <ryabinin.a.a@...il.com>,
        "glider@...gle.com" <glider@...gle.com>
Subject: Re: [PATCHv3 4/8] x86/mm: Handle LAM on context switch

On Fri, Jun 10, 2022 at 11:55:02PM +0000, Edgecombe, Rick P wrote:
> On Fri, 2022-06-10 at 17:35 +0300, Kirill A. Shutemov wrote:
> > @@ -687,6 +716,7 @@ void initialize_tlbstate_and_flush(void)
> >         struct mm_struct *mm = this_cpu_read(cpu_tlbstate.loaded_mm);
> >         u64 tlb_gen = atomic64_read(&init_mm.context.tlb_gen);
> >         unsigned long cr3 = __read_cr3();
> > +       u64 lam = cr3 & (X86_CR3_LAM_U48 | X86_CR3_LAM_U57);
> >  
> >         /* Assert that CR3 already references the right mm. */
> >         WARN_ON((cr3 & CR3_ADDR_MASK) != __pa(mm->pgd));
> > @@ -700,7 +730,7 @@ void initialize_tlbstate_and_flush(void)
> >                 !(cr4_read_shadow() & X86_CR4_PCIDE));
> >  
> >         /* Force ASID 0 and force a TLB flush. */
> > -       write_cr3(build_cr3(mm->pgd, 0));
> > +       write_cr3(build_cr3(mm->pgd, 0, lam));
> >  
> 
> Can you explain why to keep the lam bits that were in CR3 here? It
> seems to be worried some CR3 bits got changed and need to be set to a
> known state. Why not take them from the MM?
> 
> Also, it warns if the cr3 pfn doesn't match the mm pgd, should it warn
> if cr3 lam bits don't match the MM's copy?

You are right, taking LAM mode from init_mm is more correct. And we need
to update tlbstate with the new LAM mode. 

I think both CR3 and init_mm should LAM disabled here as we are bringing
CPU up. I'll add WARN_ON().

-- 
 Kirill A. Shutemov