RE: [PATCHv4 11/30] x86/tdx: Handle in-kernel MMIO

[David Laight <David.Laight@ACULAB.COM>]

From: David Laight
> Sent: 25 February 2022 02:23
> 
> From: Dave Hansen
> > Sent: 24 February 2022 20:12
> ...
> > === Limitations of this approach ===
> >
> > > MMIO addresses can be used with any CPU instruction that accesses
> > > memory. Address only MMIO accesses done via io.h helpers, such as
> > > 'readl()' or 'writeq()'.
> >
> > Any CPU instruction that accesses memory can also be used to access
> > MMIO.  However, by convention, MMIO access are typically performed via
> > io.h helpers such as 'readl()' or 'writeq()'.
> >
> > > readX()/writeX() helpers limit the range of instructions which can trigger
> > > MMIO. It makes MMIO instruction emulation feasible. Raw access to a MMIO
> > > region allows the compiler to generate whatever instruction it wants.
> > > Supporting all possible instructions is a task of a different scope.
> >
> > The io.h helpers intentionally use a limited set of instructions when
> > accessing MMIO.  This known, limited set of instructions makes MMIO
> > instruction decoding and emulation feasible in KVM hosts and SEV guests
> > today.
> >
> > MMIO accesses are performed without the io.h helpers are at the mercy of
> > the compiler.  Compilers can and will generate a much more broad set of
> > instructions which can not practically be decoded and emulated.  TDX
> > guests will oops if they encounter one of these decoding failures.
> >
> > This means that TDX guests *must* use the io.h helpers to access MMIO.
> >
> > This requirement is not new.  Both KVM hosts and AMD SEV guests have the
> > same limitations on MMIO access.
> 
> Am I reading the last sentence correctly?
> Normally (on x86 at least) a driver can mmap() PCIe addresses directly
> into a user process.
> This lets a user process directly issue PCIe read/write bus cycles.
> These can be any instructions at all.
> I don't think we've had any issues doing that in normal VMs.

Actually we won't have been exposing PCIe devices to VMs.

> Or is this emulation only applying to specific PCIe slaves?
> 
> 	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)