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Re: [Qemu-devel] [RFC PATCH 3/4] ppc: Use split I/D mmu modes to avoid f
From: |
Benjamin Herrenschmidt |
Subject: |
Re: [Qemu-devel] [RFC PATCH 3/4] ppc: Use split I/D mmu modes to avoid flushes on interrupts |
Date: |
Sun, 19 Jul 2015 22:11:30 +1000 |
On Sun, 2015-07-19 at 13:56 +0200, Paolo Bonzini wrote:
>
> On 19/07/2015 00:20, Benjamin Herrenschmidt wrote:
> > + * For BookE, we need in theory 8 MMU modes, which would
> > + * reduce performance, so instead, we ignore msr_hv and
> > + * will flush on HV context switches. We *could* improve
> > + * things a bit if needed by using 4 and 5 as HV and flush
> > + * only when HV mode changes AS but that complicates things
> > + * as we would need to remember which is the current AS mode
> > + * for HV for I and D and split more would be hell.
> > + *
>
> 8 MMU modes wouldn't reduce performance, only 9 would:
Ok, I assumed incorrectly that 8 was too much based on your changeset
comment:
<<
At 8k per TLB (for 64-bit host or target), 8 or more modes
make the TLBs bigger than 64k, and some RISC TCG backends do
not like that. On the affected hosts, cut the TLB size in
half---there is still a measurable speedup on PPC with the
next patch.
>>
IE, you wrote "8 or more".
I can easily fold back guest vs. HV into BookE, though we don't
generally support BookE HV mode anyway in TCG so there's no big hurry in
doing so (we need to add support for the shadow SPRs and a bunch of
other things for that to work).
Cheers,
Ben.
> #define CPU_TLB_BITS \
> MIN(8, \
> TCG_TARGET_TLB_DISPLACEMENT_BITS - CPU_TLB_ENTRY_BITS - \
> (NB_MMU_MODES <= 1 ? 0 : \
> NB_MMU_MODES <= 2 ? 1 : \
> NB_MMU_MODES <= 4 ? 2 : \
> NB_MMU_MODES <= 8 ? 3 : 4))
>
> Paolo
- [Qemu-devel] [RFC PATCH 1/4] ppc: Remove MMU_MODEn_SUFFIX definitions, Benjamin Herrenschmidt, 2015/07/18
- [Qemu-devel] [RFC PATCH 4/4] ppc: Do some batching of TCG tlb flushes, Benjamin Herrenschmidt, 2015/07/18
- [Qemu-devel] [RFC PATCH 3/4] ppc: Use split I/D mmu modes to avoid flushes on interrupts, Benjamin Herrenschmidt, 2015/07/18
- Re: [Qemu-devel] [RFC PATCH 3/4] ppc: Use split I/D mmu modes to avoid flushes on interrupts, Aurelien Jarno, 2015/07/19
- Re: [Qemu-devel] [RFC PATCH 3/4] ppc: Use split I/D mmu modes to avoid flushes on interrupts, Benjamin Herrenschmidt, 2015/07/19
- Re: [Qemu-devel] [RFC PATCH 3/4] ppc: Use split I/D mmu modes to avoid flushes on interrupts, Aurelien Jarno, 2015/07/20
- Re: [Qemu-devel] [RFC PATCH 3/4] ppc: Use split I/D mmu modes to avoid flushes on interrupts, Benjamin Herrenschmidt, 2015/07/20
[Qemu-devel] [RFC PATCH 2/4] tlb: Add "ifetch" argument to cpu_mmu_index(), Benjamin Herrenschmidt, 2015/07/18