Re: Problems about Stage-2 translation

[Marc Zyngier <maz@kernel.org>]

On Sat, 16 Oct 2021 13:21:01 +0100,
Chenxu Wang <irakatz51@gmail.com> wrote:
> 
> Hi all,
> 
> I am facing two problems with the Stage-2 translation in KVM.
> 
> (1)
> I reserve a memory region (e.g., 0xa000_0000 ~ 0xa020_0000) from DRAM space
> of HostOS, and fill it with some values. Then ask a GuestVM to access this
> region.
> 
> void __iomem *region=ioremap(0xa0000000, 0x1000);
> uint32_t val=ioread32(region);
> 
> But I get value 0, which is not the expected value.

It is the expected behaviour.

> I guess the reason is that the Stage-2 translation of the GuestVM does not
> map the reserved region, but it should return a translation fault, rather
> than 0. So I feel confused, could you provide some helpful explanation?

Why should the guest see a translation fault? After all, the whole
point of KVM is to transparently handle the translation fault and map
a page on demand.

> (2)
> I consider mapping a region with the Stage-2 translation. I find that KVM
> create the stage-2 table with kvm_alloc_stage2_pgd(). But I could not find
> a function that can "arbitrarily add a stage-2 mapping to the physical
> address in HostOS" when the VM is activated.
> 
> Could you provide such functions?

Follow the translation fault path in KVM, and you will find what you
are asking for.

	M.

-- 
Without deviation from the norm, progress is not possible.
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