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

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

[Fabiano Rosas]

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
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) {
+        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,
-- 
2.25.4

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

[Greg Kurz]

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,

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

[Fabiano Rosas]

Fabiano Rosas <farosas@linux.ibm.com> writes:

> 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
> 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.
>

I forgot to add a:

Reported-by: Satheesh Rajendran <sathnaga@linux.vnet.ibm.com>

> 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) {
> +        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,

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

[David Gibson]

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On Fri, Sep 11, 2020 at 01:31:23AM -0300, Fabiano Rosas 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
> 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>

Applied to ppc-for-5.2.

> ---
>  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) {
> +        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,

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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