linux-kernel - Re: [RFC 09/13] x86/mm: Disable interrupts when flushing the TLB using CR3

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Message-ID: <CA+55aFxChuKFYyUtG6a+zn82JFB=9XaM6mH9V+kdYa9iEDKUzQ@mail.gmail.com>
Date:	Fri, 8 Jan 2016 15:41:10 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andy Lutomirski <luto@...nel.org>
Cc:	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>,
	Brian Gerst <brgerst@...il.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Oleg Nesterov <oleg@...hat.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [RFC 09/13] x86/mm: Disable interrupts when flushing the TLB
 using CR3

On Fri, Jan 8, 2016 at 3:15 PM, Andy Lutomirski <luto@...nel.org> wrote:
> +       /*
> +        * We mustn't be preempted or handle an IPI while reading and
> +        * writing CR3.  Preemption could switch mms and switch back, and
> +        * an IPI could call leave_mm.  Either of those could cause our
> +        * PCID to change asynchronously.
> +        */
> +       raw_local_irq_save(flags);
>         native_write_cr3(native_read_cr3());
> +       raw_local_irq_restore(flags);

This seems sad for two reasons:

 - it adds unnecessary overhead on non-pcid setups (32-bit being an
example of that)

 - on pcid setups, wouldn't invpcid_flush_single_context() be better?

So on the whole I hate it.

Why isn't this something like

        if (static_cpu_has_safe(X86_FEATURE_INVPCID)) {
                invpcid_flush_single_context();
                return;
        }
        native_write_cr3(native_read_cr3());

*without* any flag saving crud?

And yes, that means that we'd require X86_FEATURE_INVPCID in order to
use X86_FEATURE_PCID, but that seems fine.

Or is there some reason you wanted the odd flags version? If so, that
should be documented.

               Linus