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Re: [PATCH v2 7/9] tcg: mirror mapping RWX pages for iOS optional

From: Richard Henderson
Subject: Re: [PATCH v2 7/9] tcg: mirror mapping RWX pages for iOS optional
Date: Mon, 19 Oct 2020 18:27:11 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0

On 10/18/20 6:39 PM, Joelle van Dyne wrote:
> From: osy <osy86@users.noreply.github.com>
> This allows jailbroken devices with entitlements to switch the option off.
> Signed-off-by: Joelle van Dyne <j@getutm.app>
> ---

I can guess why this performs better: half the page table entries and thus half
the tlb entries required.  Which for any non-trivially sized jit arena is going
to add up. [*]

In line with my comments re patch 6, and making this feature available
everywhere (or at least non-windows), the ifdefs would go away.  I might also
suggest default on for CONFIG_DEBUG_TCG and otherwise default off (when the
host os allows).


* Which makes me wonder how much we should use the "const TranslationBlock *"
version of that structure in the rx mapping, so that we're using a tlb entry
that is more likely to be present, since we've just branched from the code (or
just about to branch to the code) on the same page.

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