Re: [Xen-devel] [PATCH 2/9] x86: limit the amount of TLB flushing in switch_cr3_cr4()

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Sep 12, 2019 at 12:11:55PM +0200, Jan Beulich wrote:
> On 12.09.2019 11:54, Roger Pau Monné  wrote:
> > On Wed, Sep 11, 2019 at 05:22:17PM +0200, Jan Beulich wrote:
> >> We really need to flush the TLB just once, if we do so with or after the
> >> CR3 write. The only case where two flushes are unavoidable is when we
> >> mean to turn off CR4.PGE (perhaps just temporarily; see the code
> >> comment).
> >>
> >> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> > 
> > Thanks, this seems to make the logic of the function easier, but I'm
> > slightly worried about the performance impact given that a full flush
> > of all PCID contexts is done instead of the previous selective flush.
> 
> I think you've misunderstood:
> 
> >> --- a/xen/arch/x86/flushtlb.c
> >> +++ b/xen/arch/x86/flushtlb.c
> >> @@ -104,82 +104,65 @@ static void do_tlb_flush(void)
> >>  void switch_cr3_cr4(unsigned long cr3, unsigned long cr4)
> >>  {
> >>      unsigned long flags, old_cr4;
> >> -    unsigned int old_pcid;
> >>      u32 t;
> >>  
> >> +    /* Throughout this function we make this assumption: */
> >> +    ASSERT(!(cr4 & X86_CR4_PCIDE) || !(cr4 & X86_CR4_PGE));
> >> +
> >>      /* This non-reentrant function is sometimes called in interrupt 
> >> context. */
> >>      local_irq_save(flags);
> >>  
> >>      t = pre_flush();
> >>  
> >>      old_cr4 = read_cr4();
> >> -    if ( old_cr4 & X86_CR4_PGE )
> >> +    ASSERT(!(old_cr4 & X86_CR4_PCIDE) || !(old_cr4 & X86_CR4_PGE));
> >> +
> >> +    /*
> >> +     * We need to write CR4 before CR3 if we're about to enable PCIDE, at 
> >> the
> >> +     * very least when the new PCID is non-zero.
> >> +     *
> >> +     * As we also need to do two CR4 writes in total when PGE is enabled 
> >> and
> >> +     * is to remain enabled, do the one temporarily turning off the bit 
> >> right
> >> +     * here as well.
> >> +     *
> >> +     * The only TLB flushing effect we depend on here is in case we move 
> >> from
> >> +     * PGE set to PCIDE set, where we want global page entries gone (and 
> >> none
> >> +     * to re-appear) after this write.
> >> +     */
> >> +    if ( !(old_cr4 & X86_CR4_PCIDE) &&
> >> +         ((cr4 & X86_CR4_PCIDE) || (cr4 & old_cr4 & X86_CR4_PGE)) )
> >>      {
> >> -        /*
> >> -         * X86_CR4_PGE set means PCID is inactive.
> >> -         * We have to purge the TLB via flipping cr4.pge.
> >> -         */
> >>          old_cr4 = cr4 & ~X86_CR4_PGE;
> >>          write_cr4(old_cr4);
> >>      }
> >> -    else if ( use_invpcid )
> >> -    {
> >> -        /*
> >> -         * Flushing the TLB via INVPCID is necessary only in case PCIDs 
> >> are
> >> -         * in use, which is true only with INVPCID being available.
> >> -         * Without PCID usage the following write_cr3() will purge the TLB
> >> -         * (we are in the cr4.pge off path) of all entries.
> >> -         * Using invpcid_flush_all_nonglobals() seems to be faster than
> >> -         * invpcid_flush_all(), so use that.
> >> -         */
> >> -        invpcid_flush_all_nonglobals();
> 
> This simply gets moved, while ...
> 
> >> -        /*
> >> -         * CR4.PCIDE needs to be set before the CR3 write below. Otherwise
> >> -         * - the CR3 write will fault when CR3.NOFLUSH is set (which is 
> >> the
> >> -         *   case normally),
> >> -         * - the subsequent CR4 write will fault if CR3.PCID != 0.
> >> -         */
> >> -        if ( (old_cr4 & X86_CR4_PCIDE) < (cr4 & X86_CR4_PCIDE) )
> >> -        {
> >> -            write_cr4(cr4);
> >> -            old_cr4 = cr4;
> >> -        }
> >> -    }
> >>  
> >>      /*
> >> -     * If we don't change PCIDs, the CR3 write below needs to flush this 
> >> very
> >> -     * PCID, even when a full flush was performed above, as we are 
> >> currently
> >> -     * accumulating TLB entries again from the old address space.
> >> -     * NB: Clearing the bit when we don't use PCID is benign (as it is 
> >> clear
> >> -     * already in that case), but allows the if() to be more simple.
> >> +     * If the CR4 write is to turn off PCIDE, we don't need the CR3 write 
> >> to
> >> +     * flush anything, as that transition is a full flush itself.
> >>       */
> >> -    old_pcid = cr3_pcid(read_cr3());
> >> -    if ( old_pcid == cr3_pcid(cr3) )
> >> -        cr3 &= ~X86_CR3_NOFLUSH;
> >> -
> >> +    if ( (old_cr4 & X86_CR4_PCIDE) > (cr4 & X86_CR4_PCIDE) )
> >> +        cr3 |= X86_CR3_NOFLUSH;
> >>      write_cr3(cr3);
> >>  
> >>      if ( old_cr4 != cr4 )
> >>          write_cr4(cr4);
> >>  
> >>      /*
> >> -     * Make sure no TLB entries related to the old PCID created between
> >> -     * flushing the TLB and writing the new %cr3 value remain in the TLB.
> >> -     *
> >> -     * The write to CR4 just above has performed a wider flush in certain
> >> -     * cases, which therefore get excluded here. Since that write is
> >> -     * conditional, note in particular that it won't be skipped if PCIDE
> >> -     * transitions from 1 to 0. This is because the CR4 write further up 
> >> will
> >> -     * have been skipped in this case, as PCIDE and PGE won't both be set 
> >> at
> >> -     * the same time.
> >> -     *
> >> -     * Note also that PGE is always clear in old_cr4.
> >> +     *  PGE  | PCIDE | flush at
> >> +     * ------+-------+------------------------
> >> +     *  0->0 | 0->0  | CR3 write
> >> +     *  0->0 | 0->1  | n/a (see 1st CR4 write)
> >> +     *  0->x | 1->0  | CR4 write
> >> +     *  x->1 | x->1  | n/a
> >> +     *  0->0 | 1->1  | INVPCID
> >> +     *  0->1 | 0->0  | CR3 and CR4 writes
> >> +     *  1->0 | 0->0  | CR4 write
> >> +     *  1->0 | 0->1  | n/a (see 1st CR4 write)
> >> +     *  1->1 | 0->0  | n/a (see 1st CR4 write)
> >> +     *  1->x | 1->x  | n/a
> >>       */
> >> -    if ( old_pcid != cr3_pcid(cr3) &&
> > 
> > You seem to have dropped all the users of cr3_pcid, I guess the
> > function is not removed because you plan to use it in other sites?
> > 
> >> -         !(cr4 & X86_CR4_PGE) &&
> >> -         (old_cr4 & X86_CR4_PCIDE) <= (cr4 & X86_CR4_PCIDE) )
> >> -        invpcid_flush_single_context(old_pcid);
> >> +    if ( cr4 & X86_CR4_PCIDE )
> >> +        invpcid_flush_all_nonglobals();
> > 
> > Isn't this going to be quite expensive compared to the single PCID
> > flushing done before? (ie: invpcid_flush_single_context vs
> > invpcid_flush_all_nonglobals)
> 
> ... the invpcid_flush_single_context() gets eliminated altogether
> (by doing the main flush _after_ the control register writes).

Oh, thanks, I've certainly missed this move, sorry.

Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

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