On Wed, Apr 21, 2021 at 12:24:22PM +0530, Ravi Bangoria wrote: > Hi Cedric, > > On 4/21/21 12:01 PM, Cédric Le Goater wrote: > > On 4/21/21 8:20 AM, Ravi Bangoria wrote: > > > Hi David, > > > > > > On 4/19/21 10:23 AM, David Gibson wrote: > > > > On Mon, Apr 12, 2021 at 05:14:33PM +0530, Ravi Bangoria wrote: > > > > > As per the PAPR, bit 0 of byte 64 in pa-features property indicates > > > > > availability of 2nd DAWR registers. i.e. If this bit is set, 2nd > > > > > DAWR is present, otherwise not. Use KVM_CAP_PPC_DAWR1 capability to > > > > > find whether kvm supports 2nd DAWR or not. If it's supported, allow > > > > > user to set the pa-feature bit in guest DT using cap-dawr1 machine > > > > > capability. Though, watchpoint on powerpc TCG guest is not supported > > > > > and thus 2nd DAWR is not enabled for TCG mode. > > > > > > > > > > Signed-off-by: Ravi Bangoria <ravi.bangoria@xxxxxxxxxxxxx> > > > > > Reviewed-by: Greg Kurz <groug@xxxxxxxx> > > > > > > > > So, I'm actually not sure if using an spapr capability is what we want > > > > to do here. The problem is that presumably the idea is to at some > > > > point make the DAWR1 capability default to on (on POWER10, at least). > > > > But at that point you'll no longer to be able to start TCG guests > > > > without explicitly disabling it. That's technically correct, since we > > > > don't implement DAWR1 in TCG, but then we also don't implement DAWR0 > > > > and we let that slide... which I think is probably going to cause less > > > > irritation on balance. > > > > > > Ok. Probably something like this is what you want? > > > > > > Power10 behavior: > > > - KVM does not support DAWR1: Boot the guest without DAWR1 > > > support (No warnings). Error out only if user tries with > > > cap-dawr1=on. > > > - KVM supports DAWR1: Boot the guest with DAWR1 support, unless > > > user specifies cap-dawr1=off. > > > - TCG guest: Ignore cap-dawr1 i.e. boot as if there is only > > > DAWR0 (Should be fixed in future while adding PowerPC watch- > > > point support in TCG mode) > > > > > > Power10 predecessor behavior: > > > - KVM guest: Boot the guest without DAWR1 support. Error out > > > if user tries with cap-dawr1=on. > > > - TCG guest: Ignore cap-dawr1 i.e. boot as if there is only > > > DAWR0 (Should be fixed in future while adding PowerPC watch- > > > point support in TCG mode) > > > > > > > I'm wondering if we're actually just better off setting the pa feature > > > > just based on the guest CPU model. TCG will be broken if you try to > > > > use it, but then, it already is. AFAIK there's no inherent reason we > > > > couldn't implement DAWR support in TCG, it's just never been worth the > > > > trouble. > > > > > > Correct. Probably there is no practical usecase for DAWR in TCG mode. > > > > What's the expected behavior ? Is it to generate a DSI if we have a DAWR > > match ? > > Yes. DSI is the main thing. But many auxiliary stuff, off the top of my > head: > - DAR needs to be set. Now, DAR value is set differently on p8 vs p10 > (not sure about p9 because there was hw bug and thus we needed to > fully disable DAWR on p9). > - DAWR matching criteria for quadword instruction are different for > p8/p9 vs p10. > - P10 supports 512 byte unaligned watchpoints but p8/p9 does not. > > Kernel is aware of these differences and thus handles these scenarios, > sometimes as special case. i.e. Qemu will need to mimic the exact hw > behavior for the specific revision of processor. I don't actually know if qemu has TCG watchpoint support on any hardware. Presumably it would mean instrumenting all the tcg loads and stores. -- 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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