RE: [PATCH 05/12] gdb, gdbserver: Use xstate_bv for target description creation on x86.

["Schimpe, Christina" <christina.schimpe@intel.com>]

> -----Original Message-----
> From: Guinevere Larsen <guinevere@redhat.com>
> Sent: Thursday, January 30, 2025 3:52 PM
> To: Schimpe, Christina <christina.schimpe@intel.com>; gdb-
> patches@sourceware.org
> Subject: Re: [PATCH 05/12] gdb, gdbserver: Use xstate_bv for target description
> creation on x86.
> 
> On 12/20/24 5:04 PM, Schimpe, Christina wrote:
> > The XSAVE features set is organized in state components, which are a
> > set of or parts of registers.  So-called XSAVE-supported features are
> 
> I think the phrasing "are a set of or parts of registers" is a bit confusing.
> 
> I would find it more readable if it said something like "are a set of registers, or
> parts of registers".
> 
> > organized using state-component bitmaps, each bit corresonding to a
> typo, corresponding.
> > single state component.
> >
> > The SDM uses the term xstate_bv for a state-component bitmap, which is
> > defined as XCR0 | IA32_XSS.  The control register XCR0 only contains a
> > state-component bitmap that specifies user state components, while
> > IA32_XSS contains a state-component bitmap that specifies supervisor state
> components.
> >
> > Until now, XCR0 is used as input for target description creation in GDB.
> > However, a following patch will add userspace support for the CET
> > shadow stack feature by Intel.  The CET state is configured in
> > IA32_XSS and consists of 2 state components:
> > - State component 11 used for the 2 MSRs controlling user-mode
> >    functionality for CET (CET_U state)
> > - State component 12 used for the 3 MSRs containing shadow-stack pointers
> >    for privilege levels 0-2 (CET_S state).
> >
> > Reading the CET shadow stack pointer register on linux requires a
> > separate ptrace call using NT_X86_SHSTK.  To pass the CET shadow stack
> > enablement state we would like to pass the xstate_bv value instead of
> > xcr0 for target description creation.  To prepare for that, we rename
> > the xcr0 mask values for target description creation to xstate_bv.
> > However, this patch doesn't add any functional changes in GDB.
> >
> > Future states specified in IA32_XSS such as CET will create a combined
> > xstate_bv_mask including xcr0 register value and its corresponding bit
> > in the state component bitmap.  This combined mask will then be used
> > to create the target descriptions.
> All this stuff is absolute magic to me, so I can't really give a review on the code....

Thanks again for the review. 😊 I will apply your feedback for the commit message
in the next version of this series. But for now I will wait a couple more days to get
more feedback on this. 

Christina
. 


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