linux-kernel - Speeding up VMX with GDT fixmap trickery?

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Date:   Thu, 8 Jun 2017 18:13:15 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     X86 ML <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Thomas Garnier <thgarnie@...gle.com>,
        Juergen Gross <jgross@...e.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>
Subject: Speeding up VMX with GDT fixmap trickery?

Hi all-

As promised when Thomas did his GDT fixmap work, here is a draft patch
to speed up KVM by extending it.

The downside of this patch is that it makes the fixmap significantly
larger on 64-bit systems if NR_CPUS is large (it adds 15 more pages
per CPU).  I don't know if we care at all.  It also bloats the kernel
image by 4k and wastes 4k of RAM for the entire time the system is
booted.  We could avoid the latter bit (sort of) by not mapping the
extra fixmap pages at all and handling the resulting faults somehow.
That would scare me -- now we have IRET generating #PF when running
malicious , and that way lies utter madness.

The upside is that we don't need to do LGDT after a vmexit on VMX.
LGDT is slooooooooooow.  But no, I haven't benchmarked this yet.

What do you all think?

https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/kvm&id=e249a09787d6956b52d8260b2326d8f12f768799

Andrew/Boris/Juergen: what does Xen think about setting a very high
GDT limit?  Will it let us?  Should I fix it by changing
load_fixmap_gdt() (i.e. uncommenting the commented bit) or by teaching
the Xen paravirt code to just ignore the monstrous limit?  Or is it
not a problem in the first place?

--Andy