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Re: [RFC PATCH] exec: flush the whole TLB if a watchpoint crosses a page
From: |
Richard Henderson |
Subject: |
Re: [RFC PATCH] exec: flush the whole TLB if a watchpoint crosses a page boundary |
Date: |
Tue, 2 Jun 2020 10:12:31 -0700 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.8.0 |
On 6/2/20 9:49 AM, Alex Bennée wrote:
> There is no particular reason why you can't have a watchpoint in TCG
> that covers a large chunk of the address space. We could be clever
> about it but these cases are pretty rare and we can assume the user
> will expect a little performance degradation.
>
> NB: In my testing gdb will silently squash a watchpoint like:
>
> watch (char[0x7fffffffff]) *0x0
>
> to a 4 byte watchpoint. Practically it will limit the maximum size
> based on max-value-size. However given enough of a tweak the sky is
> the limit.
>
> Reported-by: Alexander Bulekov <alxndr@bu.edu>
> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
> ---
> exec.c | 6 +++++-
> 1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/exec.c b/exec.c
> index 5162f0d12f9..851ac180fe7 100644
> --- a/exec.c
> +++ b/exec.c
> @@ -1056,7 +1056,11 @@ int cpu_watchpoint_insert(CPUState *cpu, vaddr addr,
> vaddr len,
> QTAILQ_INSERT_TAIL(&cpu->watchpoints, wp, entry);
> }
>
> - tlb_flush_page(cpu, addr);
> + if (((addr + len) & TARGET_PAGE_MASK) != (addr & TARGET_PAGE_MASK)) {
This computation will be false when adding a watchpoint for the last len bytes
in the page, and no actual page crossing is required. For this reason I prefer
in_page = -(addr | TARGET_PAGE_MASK);
if (len <= in_page)
r~
> + tlb_flush(cpu);
> + } else {
> + tlb_flush_page(cpu, addr);
> + }
>
> if (watchpoint)
> *watchpoint = wp;
>