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Re: [PATCH] spapr: Handle HPT allocation failure in nested guest

From: Greg Kurz
Subject: Re: [PATCH] spapr: Handle HPT allocation failure in nested guest
Date: Fri, 11 Sep 2020 10:25:00 +0200

On Fri, 11 Sep 2020 01:31:23 -0300
Fabiano Rosas <farosas@linux.ibm.com> wrote:

> The nested KVM code does not yet support HPT guests. Calling the
> KVM_CAP_PPC_ALLOC_HTAB ioctl currently leads to KVM setting the guest
> as HPT and erroneously executing code in L1 that should only run in
> hypervisor mode, leading to an exception in the L1 vcpu thread when it
> enters the nested guest.
> This can be reproduced with -machine max-cpu-compat=power8 in the L2
> guest command line.
> The KVM code has since been modified to fail the ioctl when running in

Well, this isn't technically true for now. The KVM patch hasn't been merged
yet, but I guess it's okay to merge the QEMU patch anyway since it shouldn't
break older KVMs.

> a nested environment so QEMU needs to be able to handle that. This
> patch provides an error message informing the user about the lack of
> support for HPT in nested guests.
> Signed-off-by: Fabiano Rosas <farosas@linux.ibm.com>
> ---
>  hw/ppc/spapr.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> index 9bce1892b5..ea2c755310 100644
> --- a/hw/ppc/spapr.c
> +++ b/hw/ppc/spapr.c
> @@ -1483,6 +1483,12 @@ void spapr_reallocate_hpt(SpaprMachineState *spapr, 
> int shift,
>      spapr_free_hpt(spapr);
>      rc = kvmppc_reset_htab(shift);
> +
> +    if (rc == -EOPNOTSUPP) {

As noted on the kvm-ppc list, from a POSIX standpoint it seems that
ENOTSUP would be more appropriate... but since linux only knows
about EOPNOTSUPP and it has an unrelated and poorly named ENOTSUPP,
I guess it is okay to keep EOPNOTSUPP for the sake of consistency.

Reviewed-by: Greg Kurz <groug@kaod.org>

> +        error_setg(errp, "HPT not supported in nested guests");
> +        return;
> +    }
> +
>      if (rc < 0) {
>          /* kernel-side HPT needed, but couldn't allocate one */
>          error_setg_errno(errp, errno,

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