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Subject: Re: [RFC KVM 00/27] KVM Address Space Isolation
From:   Liran Alon <liran.alon@oracle.com>
In-Reply-To: <20190514072941.GG2589@hirez.programming.kicks-ass.net>
Date:   Tue, 14 May 2019 10:57:29 +0300
Cc:     Andy Lutomirski <luto@kernel.org>,
        Alexandre Chartre <alexandre.chartre@oracle.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Radim Krcmar <rkrcmar@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        kvm list <kvm@vger.kernel.org>, X86 ML <x86@kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        jan.setjeeilers@oracle.com, Jonathan Adams <jwadams@google.com>
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To:     Peter Zijlstra <peterz@infradead.org>
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> On 14 May 2019, at 10:29, Peter Zijlstra <peterz@infradead.org> wrote:
>=20
>=20
> (please, wrap our emails at 78 chars)
>=20
> On Tue, May 14, 2019 at 12:08:23AM +0300, Liran Alon wrote:
>=20
>> 3) =46rom (2), we should have theoretically deduced that for every
>> #VMExit, there is a need to kick the sibling hyperthread also outside
>> of guest until the #VMExit is completed.
>=20
> That's not in fact quite true; all you have to do is send the IPI.
> Having one sibling IPI the other sibling carries enough guarantees =
that
> the receiving sibling will not execute any further guest instructions.
>=20
> That is, you don't have to wait on the VMExit to complete; you can =
just
> IPI and get on with things. Now, this is still expensive, But it is
> heaps better than doing a full sync up between siblings.
>=20

I agree.

I didn=E2=80=99t say you need to do full sync. You just need to IPI the =
sibling
hyperthreads before switching to the full kernel address space.
But you need to make sure these sibling hyperthreads don=E2=80=99t get =
back into
the guest until all hyperthreads are running with KVM isolated address =
space.

It is still very expensive if done for every #VMExit. Which as I =
explained,
can be avoided in case we use the KVM isolated address space technique.

-Liran