On Thu, Jun 21, 2018 at 02:50:32PM +0200, Cédric Le Goater wrote:
> On 06/21/2018 01:52 PM, David Gibson wrote:
> > On Thu, Jun 21, 2018 at 09:01:27AM +0200, Cédric Le Goater wrote:
> >> On 06/18/2018 08:36 AM, David Gibson wrote:
> >>> KVM HV has some limitations (deriving from the hardware) that mean not all
> >>> host-cpu supported pagesizes may be usable in the guest. At present this
> >>> means that KVM guests and TCG guests may see different available page
> >>> sizes
> >>> even if they notionally have the same vcpu model. This is confusing and
> >>> also prevents migration between TCG and KVM.
> >>>
> >>> This patch makes the environment consistent by always allowing the same
> >>> set
> >>> of pagesizes. Since we can't remove the KVM limitations, we do this by
> >>> always applying the same limitations it has, even to TCG guests.
> >>>
> >>> Signed-off-by: David Gibson <da...@gibson.dropbear.id.au>
> >>>
> >>> ---
> >>> hw/ppc/spapr_caps.c | 33 +++++++++++++++++++++++++++++++++
> >>> 1 file changed, 33 insertions(+)
> >>>
> >>> diff --git a/hw/ppc/spapr_caps.c b/hw/ppc/spapr_caps.c
> >>> index 9fc739b3f5..0584c7c6ab 100644
> >>> --- a/hw/ppc/spapr_caps.c
> >>> +++ b/hw/ppc/spapr_caps.c
> >>> @@ -334,6 +334,38 @@ static void
> >>> cap_hpt_maxpagesize_apply(sPAPRMachineState *spapr,
> >>> spapr_check_pagesize(spapr, qemu_getrampagesize(), errp);
> >>> }
> >>>
> >>> +static bool spapr_pagesize_cb(void *opaque, uint32_t seg_pshift,
> >>> uint32_t pshift)
> >>> +{
> >>> + unsigned maxshift = *((unsigned *)opaque);
> >>> +
> >>> + assert(pshift >= seg_pshift);
> >>
> >> you could check that elsewhere.
> >
> > Um.. I'm not sure what you're getting at.
>
> you could put the assert in ppc_hash64_filter_pagesizes(), that is where
> the parameters are coming from.
Yes.. but it's here that we're relying on that fact. That's kind of
the point with assert()s.
>
> >>> + /* Don't allow the guest to use pages bigger than the configured
> >>> + * maximum size */
> >>> + if (pshift > maxshift) {
> >>> + return false;
> >>> + }
> >>> +
> >>> + /* For whatever reason, KVM doesn't allow multiple pagesizes
> >>> + * within a segment, *except* for the case of 16M pages in a 4k or
> >>> + * 64k segment. Always exclude other cases, so that TCG and KVM
> >>> + * guests see a consistent environment */
> >>> + if ((pshift != seg_pshift) && (pshift != 24)) {
> >>> + return false;
> >>> + }
> >
> > Note the stanza above, I'll refer to it below.
>
> ok.
>
> >
> >>> +
> >>> + return true;
> >>> +}
> >>
> >> So, do we really need ppc_hash64_filter_pagesizes() to have a callback ?
> >
> > I agree that it seems overly involved, but it was the best way I could
> > see to logically separate the TCG / softmmu specific logic from the
> > spapr specific logic.
>
> ok. I agree then.
>
> Reviewed-by: Cédric Le Goater <c...@kaod.org>
>
> Thanks,
>
> C.
>
> >> It seems that we only use the routine once in the patchset and that the
> >> only thing we need to check is 'maxshift'.
> >
> > Not quite. An earlier draft had this routine just take a max page
> > size and clamp accordingly. But that failed when I wrote the code to
> > check against the KVM capabilities, because KVM also excludes some
> > other pagesize combinations. That's what the stanza I point out above
> > is about
> >
> >> Do you envision other usage of the routine ?
> >
> > Not really, no.
> >
> >>
> >> Thanks,
> >>
> >> C.
> >>
> >>> +static void cap_hpt_maxpagesize_cpu_apply(sPAPRMachineState *spapr,
> >>> + PowerPCCPU *cpu,
> >>> + uint8_t val, Error **errp)
> >>> +{
> >>> + unsigned maxshift = val;
> >>> +
> >>> + ppc_hash64_filter_pagesizes(cpu, spapr_pagesize_cb, &maxshift);
> >>> +}
> >>> +
> >>> sPAPRCapabilityInfo capability_table[SPAPR_CAP_NUM] = {
> >>> [SPAPR_CAP_HTM] = {
> >>> .name = "htm",
> >>> @@ -401,6 +433,7 @@ sPAPRCapabilityInfo capability_table[SPAPR_CAP_NUM] =
> >>> {
> >>> .set = spapr_cap_set_pagesize,
> >>> .type = "int",
> >>> .apply = cap_hpt_maxpagesize_apply,
> >>> + .cpu_apply = cap_hpt_maxpagesize_cpu_apply,
> >>> },
> >>> };
> >>>
> >>>
> >>
> >
>
--
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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