[Bug 213257] New: KVM-PR: FPU is broken when single-stepping

[Bug 213257] New: KVM-PR: FPU is broken when single-stepping

[bugzilla-daemon]

https://bugzilla.kernel.org/show_bug.cgi?id=213257

            Bug ID: 213257
           Summary: KVM-PR: FPU is broken when single-stepping
           Product: Virtualization
           Version: unspecified
    Kernel Version: 5.6
          Hardware: PPC-64
                OS: Linux
              Tree: Mainline
            Status: NEW
          Severity: normal
          Priority: P1
         Component: kvm
          Assignee: virtualization_kvm@kernel-bugs.osdl.org
          Reporter: cand@gmx.com
        Regression: No

The FPU is completely broken when single-stepping on KVM-PR. Registers stay
zeroes, computation results are zeroes. If I disable single-stepping,
computation results are correct, but of course then I cannot dump FPRs between
every instruction.

HW is POWER9, 18-core Talos II.

5.6 is slightly old, but there are no commits under arch/powerpc/kvm since that
mention single stepping.

Program:
https://git.libre-soc.org/?p=kvm-minippc.git;a=summary

Test file:
https://ftp.libre-soc.org/mini-float-test-kvm.bin

Repro instructions:
git clone https://git.libre-soc.org/git/kvm-minippc.git
cd kvm-minippc
make

wget https://ftp.libre-soc.org/mini-float-test-kvm.bin
./kvm-minippc -i mini-float-test-kvm.bin -t trace
less trace
# you will see FPRs stay zeroes. In larger test programs that dump the computed
memory, that is wrong too (all zero).

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[Bug 213257] KVM-PR: FPU is broken when single-stepping

[bugzilla-daemon]

https://bugzilla.kernel.org/show_bug.cgi?id=213257

Luke Kenneth Casson Leighton (lkcl@lkcl.net) changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |lkcl@lkcl.net

--- Comment #1 from Luke Kenneth Casson Leighton (lkcl@lkcl.net) ---
the module being used is kvm_pr not kvm_hv.

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[Bug 213257] KVM-PR: FPU is broken when single-stepping

[bugzilla-daemon]

https://bugzilla.kernel.org/show_bug.cgi?id=213257

--- Comment #2 from Luke Kenneth Casson Leighton (lkcl@lkcl.net) ---
out of interest if the bottom half of vs0-vs31 is read (or written),
are those also zero?

completely unrelated i am running into a gdb machine-interface bug
which has been "solved" through the workaround of reading/writing
the lower 64 bits of VSX registers instead of fp0-fp31

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[Bug 213257] KVM-PR: FPU is broken when single-stepping

[bugzilla-daemon]

https://bugzilla.kernel.org/show_bug.cgi?id=213257

--- Comment #3 from Lauri Kasanen (cand@gmx.com) ---
That's probably not worth trying, since the computation results were
also wrong. The computation does not rely on register setting, it
loads, computes and saves entirely in code.

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[Bug 213257] KVM-PR: FPU is broken when single-stepping

[bugzilla-daemon]

https://bugzilla.kernel.org/show_bug.cgi?id=213257

--- Comment #4 from Luke Kenneth Casson Leighton (lkcl@lkcl.net) ---
(In reply to Lauri Kasanen from comment #3)
> That's probably not worth trying, since the computation results were
> also wrong.

ok. and the initial setup starts from a blank FPU, no host modification of FPRs
needed

> The computation does not rely on register setting, it
> loads, computes and saves entirely in code.

i realised i just said this exact thing, above, in a different way.

and is it the case that the minippc kvm host program is not attempting to read
(or write) the FPR regfile after each guest singlestep?

not that i would expect reading of the FPR regfile from the host to affect the
guest, but it's worth confirming.

trying to think, how else can this be debugged, to find out what's going on?

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