linux-kernel - Re: [RFC KVM 18/27] kvm/isolation: function to copy page table entries for percpu buffer

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Date:   Tue, 14 May 2019 13:27:11 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Alexandre Chartre <alexandre.chartre@...cle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krcmar <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        kvm list <kvm@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        jan.setjeeilers@...cle.com, Liran Alon <liran.alon@...cle.com>,
        Jonathan Adams <jwadams@...gle.com>
Subject: Re: [RFC KVM 18/27] kvm/isolation: function to copy page table
 entries for percpu buffer

On Tue, May 14, 2019 at 10:05 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Tue, May 14, 2019 at 06:24:48PM +0200, Alexandre Chartre wrote:
> > On 5/14/19 5:23 PM, Andy Lutomirski wrote:
>
> > > How important is the ability to enable IRQs while running with the KVM
> > > page tables?
> > >
> >
> > I can't say, I would need to check but we probably need IRQs at least for
> > some timers. Sounds like you would really prefer IRQs to be disabled.
> >
>
> I think what amluto is getting at, is:
>
> again:
>         local_irq_disable();
>         switch_to_kvm_mm();
>         /* do very little -- (A) */
>         VMEnter()
>
>                 /* runs as guest */
>
>         /* IRQ happens */
>         WMExit()
>         /* inspect exit raisin */
>         if (/* IRQ pending */) {
>                 switch_from_kvm_mm();
>                 local_irq_restore();
>                 goto again;
>         }
>

What I'm getting at is that running the kernel without mapping the
whole kernel is a horrible, horrible thing to do.  The less code we
can run like that, the better.