linux-kernel - Re: [PATCH] RISC-V: Issue a local tlb flush if possible.

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Date:   Tue, 13 Aug 2019 07:30:27 -0700
From:   "hch@...radead.org" <hch@...radead.org>
To:     Atish Patra <Atish.Patra@....com>
Cc:     "hch@...radead.org" <hch@...radead.org>,
        "aou@...s.berkeley.edu" <aou@...s.berkeley.edu>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        Anup Patel <Anup.Patel@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "alexios.zavras@...el.com" <alexios.zavras@...el.com>,
        "palmer@...ive.com" <palmer@...ive.com>,
        "paul.walmsley@...ive.com" <paul.walmsley@...ive.com>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "allison@...utok.net" <allison@...utok.net>
Subject: Re: [PATCH] RISC-V: Issue a local tlb flush if possible.

On Tue, Aug 13, 2019 at 12:15:15AM +0000, Atish Patra wrote:
> I thought if it recieves an empty cpumask, then it should at least do a
> local flush is the expected behavior. Are you saying that we should
> just skip all and return ? 

How could we ever receive an empty cpu mask?  I think it could only
be empty without the current cpu.  At least that is my reading of
the callers and a few other implementations.

> > 	if (!cpumask || cpumask_test_cpu(cpu, cpumask) {
> > 		if ((start == 0 && size == -1) || size > PAGE_SIZE)
> > 			local_flush_tlb_all();
> > 		else if (size == PAGE_SIZE)
> > 			local_flush_tlb_page(start);
> > 		cpumask_clear_cpu(cpuid, cpumask);
> 
> This will modify the original cpumask which is not correct. clear part
> has to be done on hmask to avoid a copy.

Indeed.  But looking at the x86 tlbflush implementation it seems like we
could use cpumask_any_but() to avoid having to modify the passed in
cpumask.