Re: [XEN PATCH] xen: Drop logic for old clang versions.

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Aug 11, 2025 at 11:37:46AM +0200, Nicola Vetrini wrote:
> On 2025-08-11 09:47, Roger Pau Monné wrote:
> > On Sun, Aug 10, 2025 at 03:03:53PM +0200, nicola.vetrini@xxxxxxxxx
> > wrote:
> > > From: Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>
> > > 
> > > The enforced toolchain baseline for clang is version 11,
> > > therefore this logic is effectively dead code.
> > > 
> > > No functional change.
> > > 
> > > Signed-off-by: Nicola Vetrini <nicola.vetrini@xxxxxxxxx>
> > 
> > Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> > 
> > > ---
> > > Mentioned in https://gitlab.com/xen-project/xen/-/issues/201
> > > ---
> > >  xen/common/coverage/llvm.c   | 4 ----
> > >  xen/include/xen/self-tests.h | 9 +--------
> > >  2 files changed, 1 insertion(+), 12 deletions(-)
> > > 
> > > diff --git a/xen/common/coverage/llvm.c b/xen/common/coverage/llvm.c
> > > index 50d7a3c5d301..517b2aa8c202 100644
> > > --- a/xen/common/coverage/llvm.c
> > > +++ b/xen/common/coverage/llvm.c
> > > @@ -44,12 +44,8 @@
> > >      ((uint64_t)'f' << 16) | ((uint64_t)'R' << 8)  | ((uint64_t)129)
> > >  #endif
> > > 
> > > -#if __clang_major__ >= 4 || (__clang_major__ == 3 &&
> > > __clang_minor__ >= 9)
> > >  #define LLVM_PROFILE_VERSION    4
> > >  #define LLVM_PROFILE_NUM_KINDS  2
> > > -#else
> > > -#error "clang version not supported with coverage"
> > > -#endif
> > 
> > Rant: most of the LLVM coverage stuff is already kind of dead code, as
> > the format of the data changes between versions and there's no way for
> > LLVM to generate the blob itself using a builtin function or
> > similar.  We haven't kept up with new formats, and now it's not
> > possible to parse the output coverage data when using newish LLVM
> > versions.
> > 
> > Linux converts it's LLVM coverage data to gcov format (which AFAIK is
> > way more stable), and exports it in gcov format.  We should consider
> > importing that from Linux.
> > 
> > Thanks, Roger.
> 
> Fair point. Generally I found coverage reports generated using LLVM tooling
> a bit more informative than gcov's, but I don't know how much work would it
> be to adapt Xen for newer versions of LLVM.

It's a pain because they keep changing the blob format between
versions, so we would basically need code in Xen to be able to
generate the right blob for each possible clang version.

It would be helpful if clang provided the helpers to generate the
coverage data in a version agnostic way, but so far I haven't found a
way to do it.  I've raised a question with upstream LLVM project, but
no replies:

https://github.com/llvm/llvm-project/issues/123034

Regards, Roger.