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Date:   Tue, 18 May 2021 17:11:58 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Andi Kleen <ak@linux.intel.com>
Cc:     Dave Hansen <dave.hansen@intel.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Andy Lutomirski <luto@kernel.org>,
        Tony Luck <tony.luck@intel.com>,
        Kirill Shutemov <kirill.shutemov@linux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@kernel.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Raj Ashok <ashok.raj@intel.com>, linux-kernel@vger.kernel.org
Subject: Re: [RFC v2-fix 1/1] x86/tdx: Handle in-kernel MMIO
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References: <3e9a26c3-8eee-88f5-f8e2-8a2dd2c028ea@intel.com>
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 <36cd2665-6d8b-9c0b-eec1-25152dcca2a3@intel.com>
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On Tue, May 18, 2021, Andi Kleen wrote:
> On 5/18/2021 8:00 AM, Dave Hansen wrote:
> > That sounds like something objective we can measure.  Does this cost 1
> > byte of extra text per readl/writel?  10?  100?
> 
> Alternatives are at least a pointer, but also the extra alternative code.
> It's definitely more than 10, I would guess 40+

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.