From mboxrd@z Thu Jan 1 00:00:00 1970 From: panand@redhat.com (Pratyush Anand) Date: Wed, 8 Jun 2016 13:43:17 +0530 Subject: Support for unaligned watchpoints in arm/arm64 In-Reply-To: References: <20160531123827.GF24936@arm.com> <20160607065520.GD13643@dhcppc6> <20160607151722.GH13643@dhcppc6> Message-ID: <20160608081317.GA24802@dhcppc6> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org Hi Pavel, On 07/06/2016:05:15:03 PM, Pavel Labath wrote: > On 7 June 2016 at 08:17, Pratyush Anand wrote: > > On 07/06/2016:12:25:20 PM, Pratyush Anand wrote: > >> On 31/05/2016:01:38:27 PM, Will Deacon wrote: > >> > On Thu, May 26, 2016 at 05:04:46PM +0100, Pavel Labath wrote: > >> > > Hello all, > >> > > >> > Hi Pavel, > >> > > >> > > I've been wondering if there are any plans about adding support for > >> > > unaligned watchpoints to the kernel. It seems quite a shame that > >> > > applications are not able not use them, even though the hardware > >> > > should support that feature. > >> > > >> > I'm actually coming round to the idea of ditching the perf hw_breakpoint > >> > mechanism entirely and simply writing a ptrace back-end that can expose > >> > the hardware features directly to userspace. The two issues with this > >> > are: > >> > > >> > (1) It's a fair amount of work > >> > >> So, by the time this new interface would come, probably we can consider a fixup > >> like following to resolve this issue at hand. Probably, things should work by > >> just allowing hw_breakpoint.c to pass checks for unaligned offset when it is a > >> WATCHPOINT, no? > > > > Sorry, this is not going to work. > > There could be some BAS values where few consecutive LSBs are 0s. > > > > Thank you for the replies. > > I would definitely welcome a more immediate fix to this issue, > although I can appreciate the reluctance to add ugly fixups to make > this work. > > Pratyush, building on your proposed fix, I think we could extend it to > make it work for watching values which are not word-aligned by > tweaking perf_event_attr.bp_addr field. E.g. if the BAS value is > 00111100 then we can set bp_addr to "aligned_addr + 2" and bp_len to I think, DBGWVR will receive only aligned address. > 4. From an interface point of view I think this would be a nice > solution. ptrace userspace user would set these breakpoints just like > it would do when programming a real register, and the contents of the > struct perf_event_attr will make perfect sense: "we have set a > watchpoint at bp_addr with length bp_len". The only problem is that > the code for converting between the perf_event_attr and hardware > register representations will be quite ugly. Also, I don't know if Yes, will have to adjust programming of both DBGWVR and DBGWCR if bp_addr is unaligned. > this will cause a problem with any other users of perf_event_attr if > they expect bp_addr field to be word-aligned. > > If you would be willing to consider such a patch, I can take a stab at > writing it. What do you think? I am not the right person to answer this. Will Deacon is. Thanks for your willingness to help on it :-) ~Pratyush