[PATCH] RISC-V: KVM: Allow to downgrade HGATP mode via SATP mode

[Guo Ren]

On Wed, Nov 26, 2025 at 2:16 AM Radim Krčmář <rkrcmar at ventanamicro.com>
wrote:

> 2025-11-25T22:18:11+08:00, <fangyu.yu at linux.alibaba.com>:
> >>> On Sat, Nov 22, 2025 at 3:50 PM <fangyu.yu at linux.alibaba.com> wrote:
> >>> >
> >>> > From: Fangyu Yu <fangyu.yu at linux.alibaba.com>
> >>> >
> >>> > Currently, HGATP mode uses the maximum value detected by the hardware
> >>> > but often such a wide GPA is unnecessary, just as a host sometimes
> >>> > doesn't need sv57.
> >>> > It's likely that no additional parameters (like no5lvl and no4lvl)
> are
> >>> > needed, aligning HGATP mode to SATP mode should meet the requirements
> >>> > of most scenarios.
> >>> Yes, no5/4lvl is not clear about satp or hgatp. So, covering HGPATP is
> >>> reasonable.
> >>
> >>The documentation should be improved, but I don't think we want to state
> >>that these parameters apply to both s- and g-stage. If we need parameters
> >>to dictate KVM behavior (g-stage management), then we should add KVM
> >>module parameters.
> >
> > Right, adding new parameters for g-stage management is clear.
> >
> > Or we could discuss this topic, from a virtual machine perspective,
> > it may not be necessary to provide all hardware configuration
> > combinations. For example, when SATP is configured as sv48,
> > configuring HGATP as sv57*4 is not very meaningful, Because the
> > VM cannot actually use more than 48 bits of GPA range.
>
> The choice of hgatp mode depends on how users configure guest's memory
> map, regardless of what satp or vsatp modes are.
> (All RV64 SvXY modes map XY bit VA to 56 bit PA.)
>
> If the machine model maps memory with set bit 55, then KVM needs to
> configure Sv57x4, and if nothing is mapped above 2 TiB, then KVM is
> completely fine with Sv39x4.
>
> A module parameter works, but I think it would be nicer to set the hgatp
> mode per-VM, because most VMs could use the efficient Sv39x4, while it's
> not a good idea to pick it as the default.
> I think KVM has enough information to do it automatically (and without
> too much complexity) by starting with Sv39x4, and expanding as needed.
>
Good point; if only a 128GB GPA memory region is needed, there is no need
for Sv57x4, which costs PTW cycles.

So, the detection should start from Sv39x4 to Sv57x4 with the guest memory
size.

-- 
Best Regards
 Guo Ren
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