linux-kernel - Re: [RFC v2-fix 1/1] x86/tdx: Handle in-kernel MMIO

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Date:   Tue, 18 May 2021 18:22:11 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Tony Luck <tony.luck@...el.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Raj Ashok <ashok.raj@...el.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC v2-fix 1/1] x86/tdx: Handle in-kernel MMIO

On Tue, May 18, 2021, Andi Kleen wrote:
> 
> > The extra bytes for .altinstructions is very different than the extra bytes for
> > the code itself.  The .altinstructions section is freed after init, so yes it
> > bloats the kernel size a bit, but the runtime footprint is unaffected by the
> > patching metadata.
> > 
> > IIRC, patching read/write{b,w,l,q}() can be done with 3 bytes of .text overhead.
> > 
> > The other option to explore is to hook/patch IO_COND(), which can be done with
> > neglible overhead because the helpers that use IO_COND() are not inlined.  In a
> > TDX guest, redirecting IO_COND() to a paravirt helper would likely cover the
> > majority of IO/MMIO since virtio-pci exclusively uses the IO_COND() wrappers.
> > And if there are TDX VMMs that want to deploy virtio-mmio, hooking
> > drivers/virtio/virtio_mmio.c directly would be a viable option.
> 
> Yes but what's the point of all that?

Patching IO_COND() is relatively low effort.  With some clever refactoring, I
suspect the net lines of code added would be less than 10.  That seems like a
worthwhile effort to avoid millions of faults over the lifetime of the guest.

> Even if it's only 3 bytes we still have a lot of MMIO all over the kernel
> which never needs it.
> 
> And I don't even see what TDX (or SEV which already does the decoding and
> has been merged) would get out of it. We handle all the #VEs just fine. And
> the instruction handling code is fairly straight forward too.
> 
> Besides instruction decoding works fine for all the existing hypervisors.
> All we really want to do is to do the same thing as KVM would do.

Heh, trust me, you don't want to do the same thing KVM does :-)