Re: [PATCH v5 0/2] MTE support for KVM guest

[Catalin Marinas]

On Wed, Dec 09, 2020 at 12:27:59PM -0600, Richard Henderson wrote:
> On 12/9/20 9:27 AM, Catalin Marinas wrote:
> > On Wed, Dec 09, 2020 at 01:25:18PM +0000, Marc Zyngier wrote:
> >> Would this syscall operate on the guest address space? Or on the VMM's
> >> own mapping?
> ...
> > Whatever is easier for the VMM, I don't think it matters as long as the
> > host kernel can get the actual physical address (and linear map
> > correspondent). Maybe simpler if it's the VMM address space as the
> > kernel can check the access permissions in case you want to hide the
> > guest memory from the VMM for other reasons (migration is also off the
> > table).
> 
> Indeed, such a syscall is no longer specific to vmm's and may be used for any
> bulk move of tags that userland might want.

For CRIU, I think the current ptrace interface would do. With VMMs, the
same remote VM model doesn't apply (the "remote" VM is actually the
guest memory). I'd keep this under a KVM ioctl() number rather than a
new, specific syscall.

> > Without syscalls, an option would be for the VMM to create two mappings:
> > one with PROT_MTE for migration and the other without for normal DMA
> > etc. That's achievable using memfd_create() or shm_open() and two mmap()
> > calls, only one having PROT_MTE. The VMM address space should be
> > sufficiently large to map two guest IPAs.
> 
> I would have thought that the best way is to use TCO, so that we don't have to
> have dual mappings (and however many MB of extra page tables that might imply).

The problem appears when the VMM wants to use MTE itself (e.g. linked
against an MTE-aware glibc), toggling TCO is no longer generic enough,
especially when it comes to device emulation.

-- 
Catalin