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 ? C.
