RE: [PATCH v4 2/2] LAM: Enable tagged pointer support for watchpoints.

["Willgerodt, Felix" <felix.willgerodt@intel.com>]

> -----Original Message-----
> From: Schimpe, Christina <christina.schimpe@intel.com>
> Sent: Donnerstag, 4. Juli 2024 12:18
> To: gdb-patches@sourceware.org
> Cc: Willgerodt, Felix <felix.willgerodt@intel.com>; luis.machado@arm.com
> Subject: [PATCH v4 2/2] LAM: Enable tagged pointer support for watchpoints.
> 
> From: Christina Schimpe <christina.schimpe@intel.com>
> 
> The Intel (R) linear address masking (LAM) feature modifies the checking
> applied to 64-bit linear addresses.  With this so-called "modified
> canonicality check" the processor masks the metadata bits in a pointer
> before using it as a linear address.  LAM supports two different modes that
> differ regarding which pointer bits are masked and can be used for
> metadata: LAM 48 resulting in a LAM width of 15 and LAM 57 resulting in a
> LAM width of 6.
> 
> This patch adjusts watchpoint addresses based on the currently enabled
> LAM mode using the untag mask provided in the /proc/<pid>/status file.
> As LAM can be enabled at runtime or as the configuration may change
> when entering an enclave, GDB checks enablement state each time a watchpoint
> is updated.
> 
> In contrast to the patch implemented for ARM's Top Byte Ignore "Clear
> non-significant bits of address on memory access", it is not necessary to
> adjust addresses before they are passed to the target layer cache, as
> for LAM tagged pointers are supported by the system call to read memory.
> Additionally, LAM applies only to addresses used for data accesses.
> Thus, it is sufficient to mask addresses used for watchpoints.
> 
> The following examples are based on a LAM57 enabled program.
> Before this patch tagged pointers were not supported for watchpoints:
> ~~~
> (gdb) print pi_tagged
> $2 = (int *) 0x10007ffffffffe004
> (gdb) watch *pi_tagged
> Hardware watchpoint 2: *pi_tagged
> (gdb) c
> Continuing.
> Couldn't write debug register: Invalid argument.
> ~~~~
> 
> Once LAM 48 or LAM 57 is enabled for the current program, GDB can now
> specify watchpoints for tagged addresses with LAM width 15 or 6,
> respectively.
> ---
>  gdb/NEWS                             |  2 +
>  gdb/amd64-linux-tdep.c               | 64 ++++++++++++++++++++++++++++
>  gdb/testsuite/gdb.arch/amd64-lam.c   | 49 +++++++++++++++++++++
>  gdb/testsuite/gdb.arch/amd64-lam.exp | 46 ++++++++++++++++++++
>  gdb/testsuite/lib/gdb.exp            | 63 +++++++++++++++++++++++++++
>  5 files changed, 224 insertions(+)
>  create mode 100755 gdb/testsuite/gdb.arch/amd64-lam.c
>  create mode 100644 gdb/testsuite/gdb.arch/amd64-lam.exp

Approved-By: Felix Willgerodt <felix.willgerodt@intel.com>

Thanks,
Felix
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