Re: [PATCH v4 2/8] KVM: arm64: Introduce pkvm_alloc_private_va_range()

[Kalesh Singh]

On Tue, Mar 1, 2022 at 11:46 PM Marc Zyngier <maz@xxxxxxxxxx> wrote:
>
> On Fri, 25 Feb 2022 03:34:47 +0000,
> Kalesh Singh <kaleshsingh@xxxxxxxxxx> wrote:
> >
> > pkvm_hyp_alloc_private_va_range() can be used to reserve private VA ranges
> > in the pKVM nVHE hypervisor (). Also update __pkvm_create_private_mapping()
> > to allow specifying an alignment for the private VA mapping.
> >
> > These will be used to implement stack guard pages for pKVM nVHE hypervisor
> > (in a subsequent patch in the series).
> >
> > Credits to Quentin Perret <qperret@xxxxxxxxxx> for the idea of moving
> > private VA allocation out of __pkvm_create_private_mapping()
> >
> > Signed-off-by: Kalesh Singh <kaleshsingh@xxxxxxxxxx>
> > ---
> >
> > Changes in v4:
> >   - Handle null ptr in pkvm_alloc_private_va_range() and replace
> >     IS_ERR_OR_NULL checks in callers with IS_ERR checks, per Fuad
> >   - Fix kernel-doc comments format, per Fuad
> >   - Format __pkvm_create_private_mapping() prototype args (< 80 col), per Fuad
> >
> > Changes in v3:
> >   - Handle null ptr in IS_ERR_OR_NULL checks, per Mark
> >
> > Changes in v2:
> >   - Allow specifying an alignment for the private VA allocations, per Marc
>
> I probably badly expressed my earlier concern.
>
> Yes, an alignment is necessary. But how often do we want an alignment
> that isn't naturally aligned to the size of the allocation (i.e. the
> power of 2 >= the size of the allocation)? This is what the rest of
> the kernel does (get_order() and co), and I thing we should follow
> this.

Hi Marc,

Thanks for clarifying. I think making the alignment implicitly based
on the size here will create unnecessary holes where PAGE_SIZE
alignment would be ok and potentially overflow the private VA space
earlier. Is it not a concern?

- Kalesh
>
> This applies to both this patch and the previous one.
>
> Thanks,
>
>         M.
>
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> Without deviation from the norm, progress is not possible.
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