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Re: [Qemu-devel] best way to implement emulation of AArch64 tagged addre

From: Richard Henderson
Subject: Re: [Qemu-devel] best way to implement emulation of AArch64 tagged addresses
Date: Mon, 4 Apr 2016 09:28:32 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.7.1

On 04/04/2016 08:51 AM, Peter Maydell wrote:
I thought of two possible ways to approach implementing this.
Option (1) would be to change the codegen in translate-a64.c so that
we mask out high bits before doing the QEMU load/store TCG op.
Option (2) leaves the VA that we pass to the TCG load/store alone
(ie with tag bits intact) and tries to handle this all in the va-to-pa

I think option (1) is a non-starter because of that requirement to
report the full address with tags in the FAR (as well as being slower
due to all the extra masking on memory operations). So that leaves
option (2), possibly with some help from common code to make things
a bit less awkward.


In particular I think if you just do the relevant handling of the tag
bits in target-arm's get_phys_addr() and its subroutines then this
should work ok, with the exceptions that:
  * the QEMU TLB code will think that [tag A + address X] and
    [tag B + address X] are different virtual addresses and they will
    miss each other in the TLB

Yep.  Not only miss, but actively contend with each other.

  * tlb invalidate by address becomes nasty because we need to invalidate
    [every tag + address X]

Hmm. We should require only one flush for X. But the common code doesn't know that... I suppose a new tlb_flush_page_mask would do the trick.

Can we fix those just by having arm_tlb_fill() call
tlb_set_page_with_attrs() with the vaddr with the tag masked out?

No, that misses when we perform the full vaddr+tag comparison on the TCG fast 

[NB: this is all assuming softmmu; getting tagged addresses to work
in linux-user mode would require doing the masking in translate.c,
but I definitely don't want two implementations so I guess we just
ignore linux-user here.]

Let's just say it's another user for the oft wished for softmmu-in-linux-user.


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