Re: [PATCH v2] x86: Quark: Add if/else to setup_arch for Quark TLB bug
From: Bryan O'Donoghue
Date: Fri Sep 26 2014 - 17:28:50 EST
On 26/09/14 21:59, Henrique de Moraes Holschuh wrote:
I'm confused, now.
Wasn't the other patch -- which just added a comment -- the one selected as
a better fix, because there is absolutely no point in calling __flush_tlb()
on Quark X1000 *right after* you just flushed the TLB [on these processors]
by doing a load_cr3() ?
Should this one ([PATCH v2] x86: Quark: Add if/else to setup_arch for Quark
TLB bug) be ignored? Or should the other one which just adds a comment be
My view is that the CR3 load should have flushed the TLB in it's entirety.
Ong Boong Leong said that a discussion he had which included HPA
concluded with a flush of the TLB being required after the CR3 reload.
The current code
a) Writes to CR3.
On Quark that will flush the entire TLB
Ref: Quark SoC X1000 Core Developer's Manual section 6.4.11
On other processors this won't invalidate the entire TLB under every
scenario. For example a PTE with PGE on may not be flushed.
Ref: Intel 64 and IA32 Architectures .. vol (3a, 3b, 3c) 325384.pdf
Depending on the value of cpu_has_pge() - which will be true for ~all
x86 processors will call
native_write_cr4(cr4 & ~X86_CR4_PGE);
According to the same section "184.108.40.206" this will nuke the rest of the
TLB entries even with PGE bits set in the PTEs
So the steps a & b are the appropriate sets to take to entirely flush
the TLB for most processors i.e. one's that support PGE
The suggestion is that on Quark we should flush the TLB again.
The documentation doesn't really support that statement but, my view is
OK if we are going to accommodate that position let's not force a third
TLB flush on everybody who isn't running a Quark.
From the technical/documentation standpoint I don't think Quark is
forcing a step c - if we do decide to add a __flush_tlb() call for the
sake of Quark anyway - then let's not foist the extra cycles on every
other x86 processor out there.
My preference is
1. Just comment the code as is to explain why it works for Quark.
If that's not good enough for people then
2. if/else the flow so that Quark does __flush_tlb() and the rest of the
world does a __flush_tlb_all()
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