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SubjectRe: [PATCH v19 02/15] arm64: Introduce prctl() options to control the tagged user addresses ABI
On Thu, Aug 01, 2019 at 09:45:05AM -0700, Dave Hansen wrote:
> On 8/1/19 5:38 AM, Kevin Brodsky wrote:
> > This patch series only changes what is allowed or not at the syscall
> > interface. It does not change the address space size. On arm64, TBI (Top
> > Byte Ignore) has always been enabled for userspace, so it has never been
> > possible to use the upper 8 bits of user pointers for addressing.
> Oh, so does the address space that's available already chop that out?

Yes. Currently the hardware only supports 52-bit virtual addresses. It
could be expanded (though it needs a 5th page table level) to 56-bit VA
but it's not currently on our (hardware) plans. Beyond 56-bit, it cannot
be done without breaking the software expectations (and hopefully I'll
retire before we need this ;)).

> > If other architectures were to support a similar functionality, then I
> > agree that a common and more generic interface (if needed) would be
> > helpful, but as it stands this is an arm64-specific prctl, and on arm64
> > the address tag is defined by the architecture as bits [63:56].
> It should then be an arch_prctl(), no?

I guess you just want renaming SET_TAGGED_ADDR_CTRL() to
arch_prctl_tagged_addr_ctrl_set()? (similarly for 'get')


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