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Re: [PATCH v4 02/10] x86/vmx: add IPT cpu feature


On 02/07/2020 14:30, Jan Beulich wrote:
On 02.07.2020 11:57, Julien Grall wrote:

On 02/07/2020 10:18, Jan Beulich wrote:
On 02.07.2020 10:54, Julien Grall wrote:

On 02/07/2020 09:50, Jan Beulich wrote:
On 02.07.2020 10:42, Julien Grall wrote:
On 02/07/2020 09:29, Jan Beulich wrote:
I'm with Andrew here, fwiw, as long as the little bit of code that
is actually put in common/ or include/xen/ doesn't imply arbitrary
restrictions on acceptable values.
Well yes the code is simple. However, the code as it is wouldn't be
usuable on other architecture without additional work (aside arch
specific code). For instance, there is no way to map the buffer outside
of Xen as it is all x86 specific.

If you want the allocation to be in the common code, then the
infrastructure to map/unmap the buffer should also be in common code.
Otherwise, there is no point to allocate it in common.

I don't think I agree here - I see nothing wrong with exposing of
the memory being arch specific, when allocation is generic. This
is no different from, in just x86, allocation logic being common
to PV and HVM, but exposing being different for both.

Are you suggesting that the way it would be exposed may be different for
other architecture?

Why not? To take a possibly extreme example - consider an arch
where (for bare metal) the buffer is specified to appear at a
fixed range of addresses.

I am probably missing something here... The current goal is the buffer
will be mapped in the dom0. Most likely the way to map it will be using
the acquire hypercall (unless you invent a brand new one...).

For a guest, you could possibly reserve a fixed range and then map it
when creating the vCPU in Xen. But then, you will likely want a fixed
size... So why would you bother to ask the user to define the size?

Because there may be the option to only populate part of the fixed

It was yet another extreme case ;).

Another way to do it, would be the toolstack to do the mapping. At which
point, you still need an hypercall to do the mapping (probably the
hypercall acquire).

There may not be any mapping to do in such a contrived, fixed-range
environment. This scenario was specifically to demonstrate that the
way the mapping gets done may be arch-specific (here: a no-op)
despite the allocation not being so.
You are arguing on extreme cases which I don't think is really helpful here. Yes if you want to map at a fixed address in a guest you may not need the acquire hypercall. But in most of the other cases (see has for the tools) you will need it.

So what's the problem with requesting to have the acquire hypercall implemented in common code?


Julien Grall



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