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

[Sean Christopherson]

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.