Re: [PATCH v5 3/5] hyperv: Enable the hypercall output page for the VTL mode

[Wei Liu <wei.liu@kernel.org>]

On Tue, Jan 07, 2025 at 03:11:15PM -0800, Roman Kisel wrote:
> 
> 
> On 1/7/2025 11:18 AM, Stanislav Kinsburskii wrote:
> > On Mon, Jan 06, 2025 at 01:07:25PM -0800, Roman Kisel wrote:
> > 
> 
> [...]
> 
> > My point is that the proposed fix looks more like an Underhill-tailored
> > bandage and doesn't take the needs of other stake holders into
> > consideration.
> The patch takes as much into consideration as present in the hyperv-next
> tree. Working on the open-source project seems to be harder otherwise.
> A bandage, or not, that's a matter of opinion. There's a been a break,
> here's the bandage.
> 
> > 
> > What is the urgency in merging of this particular change?
> 
> The get_vtl function is broken thus blocking any further work on
> upstreaming VTL mode patches, ARM64 and more. That's not an urgent
> urgency where customers are in pain, more like the urgency of needing
> to take the trash out, and until that happens, continuing inhaling the
> fumes.
> 
> The urgency of unblocking is to continue work on proposing VTL mode
> patches not to carry lots of out-of-tree code in the fork.
> 
> There might be a future where the Hyper-V code offers an API surface
> covering needs of consumers like dom0 and VTLs whereby they maybe can
> be built as an out-of-tree modules so the opinions wouldn't clash as
> much.
> 
> Avoiding using the output hypercall page leads to something like[1]
> and it looks quite complicated although that's the bare bones, lots
> of notes.
> 
> [1]
> 
> /*
>  * Fast extended hypercall with 20 bytes of input and 16 bytes of
>  * output for getting a VP register.
>  *
>  * NOTES:
>  *  1. The function is __init only atm, so the XMM context isn't
>  *     used by the user mode.
>  *  2. X86_64 only.
>  *  3. Fast extended hypercalls may use XMM0..XMM6, and XMM is
>  *     architerctural on X86_64 yet the support should be enabled
>  *     in the CR's. Here, need RDX, R8 and XMM0 for input and RDX,
>  *     R8 for output
>  *  4. No provisions for TDX and SEV-SNP for the sake of simplicity
>  *     (the hypervisor cannot see the guest registers in the
>  *     confidential VM), would need to fallback.

I am not worried about this point. There are architectural defined ways
to handle this.

>  *  5. The robust implementation would need to check if fast extended
>  *     hypercalls are available by checking the synthehtic CPUID leaves.
>  *     A separate leaf indicates fast output support.
>  *     It _almost_ certainly has to be, unless somehow disabled, hard
>  *     to see why that would be needed.
>  */

The rest I agree. Not worth the effort just to add that support here for
a single user.

I've been thinking about adding the extended hypercall support for a
while, but I'm not sure if it's worth the effort overall.

An aspiring developer who's interested in this area is building a
prototype to see if extended fast hypercall can give a boost to some of
the frequent hypercalls.

In any case, I think this patch is fine.

Thanks,
Wei.