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Re: [PATCH 1/2] exec: flush CPU TB cache in breakpoint_invalidate

From: Richard Henderson
Subject: Re: [PATCH 1/2] exec: flush CPU TB cache in breakpoint_invalidate
Date: Sun, 1 Dec 2019 15:48:55 -0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.2.1

On 11/27/19 10:06 PM, Max Filippov wrote:
> When a breakpoint is inserted at location for which there's currently no
> virtual to physical translation no action is taken on CPU TB cache. If a
> TB for that virtual address already exists but is not visible ATM the
> breakpoint won't be hit next time an instruction at that address will be
> executed.
> Flush entire CPU TB cache in breakpoint_invalidate to force
> re-translation of all TBs for the breakpoint address.
> This change fixes the following scenario:
> - linux user application is running
> - a breakpoint is inserted from QEMU gdbstub for a user address that is
>   not currently present in the target CPU TLB
> - an instruction at that address is executed, but the external debugger
>   doesn't get control.
> Signed-off-by: Max Filippov <address@hidden>
> ---
> Changes RFC->v1:
> - do tb_flush in breakpoint_invalidate unconditionally

Unlike Paolo, I don't think this is a good idea.

If I was going to change anything here, I'd change this to not use
cpu_get_phys_page_attrs_debug but using the caching available from the actual
cputlb, using cc->tlb_fill() in probe mode -- something akin to probe_access(),
but not returning a host address, nor handling watchpoints nor notdirty.

This would help flushing too much by distinguishing different tbs for the same
virtual address mapping to a different physical address.


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