Re: [PATCH v2] riscv/gdb: add virt mode debug interface

[Alex Bennée]

Yanfeng <yfliu2008@qq.com> writes:

> On Thu, 2024-11-28 at 14:21 +0000, Alex Bennée wrote:
>> Yanfeng Liu <yfliu2008@qq.com> writes:
>> 
>> > This adds `virt` virtual register on debug interface so that users
>> > can access current virtualization mode for debugging purposes.
>> > 
>> > Signed-off-by: Yanfeng Liu <yfliu2008@qq.com>
>> > ---
>> >  gdb-xml/riscv-32bit-virtual.xml |  1 +
>> >  gdb-xml/riscv-64bit-virtual.xml |  1 +
>> >  target/riscv/gdbstub.c          | 18 ++++++++++++------
>> >  3 files changed, 14 insertions(+), 6 deletions(-)
>> > 
>> > diff --git a/gdb-xml/riscv-32bit-virtual.xml b/gdb-xml/riscv-32bit-
>> > virtual.xml
>> > index 905f1c555d..d44b6ca2dc 100644
>> > --- a/gdb-xml/riscv-32bit-virtual.xml
>> > +++ b/gdb-xml/riscv-32bit-virtual.xml
>> > @@ -8,4 +8,5 @@
>> >  <!DOCTYPE feature SYSTEM "gdb-target.dtd">
>> >  <feature name="org.gnu.gdb.riscv.virtual">
>> >    <reg name="priv" bitsize="32"/>
>> > +  <reg name="virt" bitsize="32"/>
>> >  </feature>
>> > diff --git a/gdb-xml/riscv-64bit-virtual.xml b/gdb-xml/riscv-64bit-
>> > virtual.xml
>> > index 62d86c237b..7c9b63d5b6 100644
>> > --- a/gdb-xml/riscv-64bit-virtual.xml
>> > +++ b/gdb-xml/riscv-64bit-virtual.xml
>> > @@ -8,4 +8,5 @@
>> >  <!DOCTYPE feature SYSTEM "gdb-target.dtd">
>> >  <feature name="org.gnu.gdb.riscv.virtual">
>> >    <reg name="priv" bitsize="64"/>
>> > +  <reg name="virt" bitsize="64"/>
>> >  </feature>
>> 
>> I assume these are mirrored in gdb not a QEMU only extension?
>
> So far I think it is a QEMU extension and the `gdb-multiarch` doesn't treat is
> specially. My tests shows it basically works:
>
> ```
> (gdb) ir virt
> priv           0x3    prv:3 [Machine]
> virt           0x0    0
> (gdb) set $priv = 2
> (gdb) ir virt
> priv           0x1    prv:1 [Supervisor]
> virt           0x0    0
> (gdb) set $virt = 1
> (gdb) ir virt
> priv           0x1    prv:1 [Supervisor]
> virt           0x1    1
> (gdb) set $virt = 0
> (gdb) ir virt
> priv           0x1    prv:1 [Supervisor]
> virt           0x0    0
> (gdb) set $virt = 1
> (gdb) ir virt
> priv           0x1    prv:1 [Supervisor]
> virt           0x1    1
> (gdb) set $priv = 3
> (gdb) ir virt
> priv           0x3    prv:3 [Machine]
> virt           0x0    0
> ```

A gdbstub test case would be useful for this although I don't know if
the RiscV check-tcg tests switch mode at all.

>
> As I am rather new to QEMU, please teach how we can add it as a QEMU only
> extension.

You don't need to extend the XML from GDB, you can build a specific one
for QEMU extensions. For example:

    gdb_feature_builder_init(&param.builder,
                             &cpu->dyn_sysreg_feature.desc,
                             "org.qemu.gdb.arm.sys.regs",
                             "system-registers.xml",
                             base_reg);

This exports all the system registers QEMU knows about and GDB can
access generically. Note the id is org.qemu..., indicating its our
schema not gdbs.

-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro