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

["Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>]

On Tue, Jun 28, 2022 at 04:33:21PM -0700, Andy Lutomirski wrote:
> On 6/10/22 07:35, Kirill A. Shutemov wrote:
> > Linear Address Masking mode for userspace pointers encoded in CR3 bits.
> > The mode is selected per-thread. Add new thread features indicate that the
> > thread has Linear Address Masking enabled.
> > 
> > switch_mm_irqs_off() now respects these flags and constructs CR3
> > accordingly.
> > 
> > The active LAM mode gets recorded in the tlb_state.
> > 
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> > ---
> >   arch/x86/include/asm/mmu.h         |  1 +
> >   arch/x86/include/asm/mmu_context.h | 24 ++++++++++++
> >   arch/x86/include/asm/tlbflush.h    |  3 ++
> >   arch/x86/mm/tlb.c                  | 62 ++++++++++++++++++++++--------
> >   4 files changed, 75 insertions(+), 15 deletions(-)
> > 
> > diff --git a/arch/x86/include/asm/mmu.h b/arch/x86/include/asm/mmu.h
> > index 5d7494631ea9..d150e92163b6 100644
> > --- a/arch/x86/include/asm/mmu.h
> > +++ b/arch/x86/include/asm/mmu.h
> > @@ -40,6 +40,7 @@ typedef struct {
> >   #ifdef CONFIG_X86_64
> >   	unsigned short flags;
> > +	u64 lam_cr3_mask;
> >   #endif
> >   	struct mutex lock;
> > diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
> > index b8d40ddeab00..e6eac047c728 100644
> > --- a/arch/x86/include/asm/mmu_context.h
> > +++ b/arch/x86/include/asm/mmu_context.h
> > @@ -91,6 +91,29 @@ static inline void switch_ldt(struct mm_struct *prev, struct mm_struct *next)
> >   }
> >   #endif
> > +#ifdef CONFIG_X86_64
> > +static inline u64 mm_cr3_lam_mask(struct mm_struct *mm)
> > +{
> > +	return mm->context.lam_cr3_mask;
> > +}
> > +
> > +static inline void dup_lam(struct mm_struct *oldmm, struct mm_struct *mm)
> > +{
> > +	mm->context.lam_cr3_mask = oldmm->context.lam_cr3_mask;
> > +}
> > +
> > +#else
> > +
> > +static inline u64 mm_cr3_lam_mask(struct mm_struct *mm)
> > +{
> > +	return 0;
> > +}
> > +
> > +static inline void dup_lam(struct mm_struct *oldmm, struct mm_struct *mm)
> > +{
> > +}
> > +#endif
> 
> Do we really need the ifdeffery here?  I see no real harm in having the
> field exist on 32-bit -- we don't care much about performance for 32-bit
> kernels.

The waste doesn't feel right to me. I would rather keep it.

But sure I can do this if needed.

> > -	if (real_prev == next) {
> > +	if (real_prev == next && prev_lam == new_lam) {
> >   		VM_WARN_ON(this_cpu_read(cpu_tlbstate.ctxs[prev_asid].ctx_id) !=
> >   			   next->context.ctx_id);
> 
> This looks wrong to me.  If we change threads within the same mm but lam
> changes (which is certainly possible by a race if nothing else) then this
> will go down the "we really are changing mms" path, not the "we're not
> changing but we might need to flush something" path.

If LAM gets enabled we must write CR3 with the new LAM mode. Without the
change real_prev == next case will not do this for !was_lazy case.

Note that currently enabling LAM is done by setting LAM mode in the mmu
context and doing switch_mm(current->mm, current->mm, current), so it is
very important case.

-- 
 Kirill A. Shutemov