Re: [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') -- Catalin ___ dri-devel mailing list dri-devel@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/dri-devel
Re: [PATCH v19 02/15] arm64: Introduce prctl() options to control the tagged user addresses ABI
On 31/07/2019 18:05, Dave Hansen wrote:
On 7/23/19 10:58 AM, Andrey Konovalov wrote:
+long set_tagged_addr_ctrl(unsigned long arg)
+{
+ if (!tagged_addr_prctl_allowed)
+ return -EINVAL;
+ if (is_compat_task())
+ return -EINVAL;
+ if (arg & ~PR_TAGGED_ADDR_ENABLE)
+ return -EINVAL;
+
+ update_thread_flag(TIF_TAGGED_ADDR, arg & PR_TAGGED_ADDR_ENABLE);
+
+ return 0;
+}
Instead of a plain enable/disable, a more flexible ABI would be to have
the tag mask be passed in. That way, an implementation that has a
flexible tag size can select it. It also ensures that userspace
actually knows what the tag size is and isn't surprised if a hardware
implementation changes the tag size or position.
Also, this whole set deals with tagging/untagging, but there's an
effective loss of address space when you do this. Is that dealt with
anywhere? How do we ensure that allocations don't get placed at a
tagged address before this gets turned on? Where's that checking?
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.
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].
Kevin
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Re: [PATCH v19 02/15] arm64: Introduce prctl() options to control the tagged user addresses ABI
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? > 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? ___ dri-devel mailing list dri-devel@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/dri-devel
Re: [PATCH v19 02/15] arm64: Introduce prctl() options to control the tagged user addresses ABI
On 7/23/19 10:58 AM, Andrey Konovalov wrote:
> +long set_tagged_addr_ctrl(unsigned long arg)
> +{
> + if (!tagged_addr_prctl_allowed)
> + return -EINVAL;
> + if (is_compat_task())
> + return -EINVAL;
> + if (arg & ~PR_TAGGED_ADDR_ENABLE)
> + return -EINVAL;
> +
> + update_thread_flag(TIF_TAGGED_ADDR, arg & PR_TAGGED_ADDR_ENABLE);
> +
> + return 0;
> +}
Instead of a plain enable/disable, a more flexible ABI would be to have
the tag mask be passed in. That way, an implementation that has a
flexible tag size can select it. It also ensures that userspace
actually knows what the tag size is and isn't surprised if a hardware
implementation changes the tag size or position.
Also, this whole set deals with tagging/untagging, but there's an
effective loss of address space when you do this. Is that dealt with
anywhere? How do we ensure that allocations don't get placed at a
tagged address before this gets turned on? Where's that checking?
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