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

[Alex Ionescu]

For another 2c worth, I had previously requested #1 (always allocate
output page) as this would simplify some further work I was interested
in at some point to provide VSM-like functionality to a VTL 0 Linux
guest, which would, at that point, not make it wasteful for VTL 0 any
longer (since some required hypercalls for VSM support need an output
page).

I realize this is "future-proofing" something that doesn't exist yet
but it would avoid a further change down the road.

Best regards,
Alex Ionescu

Best regards,
Alex Ionescu


On Mon, Jan 6, 2025 at 9:45 AM Alex Ionescu <aionescu@xxxxxxxxx> wrote:
>
> For another 2c worth, I had previously requested #1 (always allocate output page) as this would simplify some further work I was interested in at some point to provide VSM-like functionality to a VTL 0 Linux guest, which would, at that point, not make it wasteful for VTL 0 any longer (since some required hypercalls for VSM support need an output page).
>
> I realize this is "future-proofing" something that doesn't exist yet but it would avoid a further change down the road.
>
> Best regards,
> Alex Ionescu
>
>
> On Fri, Jan 3, 2025 at 5:08 PM Michael Kelley <mhklinux@xxxxxxxxxxx> wrote:
>>
>> From: Stanislav Kinsburskii <skinsburskii@xxxxxxxxxxxxxxxxxxx> Sent: Friday, January 3, 2025 11:20 AM
>> >
>> > On Mon, Dec 30, 2024 at 10:09:39AM -0800, Roman Kisel wrote:
>> > > Due to the hypercall page not being allocated in the VTL mode,
>> > > the code resorts to using a part of the input page.
>> > >
>> > > Allocate the hypercall output page in the VTL mode thus enabling
>> > > it to use it for output and share code with dom0.
>> > >
>> > > Signed-off-by: Roman Kisel <romank@xxxxxxxxxxxxxxxxxxx>
>> > > ---
>> > >  drivers/hv/hv_common.c | 6 +++---
>> > >  1 file changed, 3 insertions(+), 3 deletions(-)
>> > >
>> > > diff --git a/drivers/hv/hv_common.c b/drivers/hv/hv_common.c
>> > > index c6ed3ba4bf61..c983cfd4d6c0 100644
>> > > --- a/drivers/hv/hv_common.c
>> > > +++ b/drivers/hv/hv_common.c
>> > > @@ -340,7 +340,7 @@ int __init hv_common_init(void)
>> > >     BUG_ON(!hyperv_pcpu_input_arg);
>> > >
>> > >     /* Allocate the per-CPU state for output arg for root */
>> > > -   if (hv_root_partition) {
>> > > +   if (hv_root_partition || IS_ENABLED(CONFIG_HYPERV_VTL_MODE)) {
>> >
>> > This check doesn't look nice.
>> > First of all, IS_ENABLED(CONFIG_HYPERV_VTL_MODE) doesn't mean that this
>> > particular kernel is being booted in VTL other that VTL0.
>>
>> Actually, it does mean that. Kernels built with CONFIG_HYPERV_VTL_MODE=y
>> will not run as a normal guest in VTL 0. See the third paragraph of the
>> "help" section for HYPERV_VTL_MODE in drivers/hv/Kconfig.
>>
>> Michael
>>
>> >
>> > Second, current approach is that a VTL1+ kernel is a different build from VTL0
>> > kernel and thus relying on the config option looks reasonable. However,
>> > this is not true in general case.
>> >
>> > I'd suggest one of the following three options:
>> >
>> > 1. Always allocate per-cpu output pages. This is wasteful for the VTL0
>> > guest case, but it might worth it for overall simplification.
>> >
>> > 2. Don't touch this code and keep the cnage in the Underhill tree.
>> >
>> > 3. Introduce a configuration option (command line or device tree or
>> > both) and use it instead of the kernel config option.
>> >
>> > Thanks,
>> > Stas
>> >
>> > >             hyperv_pcpu_output_arg = alloc_percpu(void *);
>> > >             BUG_ON(!hyperv_pcpu_output_arg);
>> > >     }
>> > > @@ -435,7 +435,7 @@ int hv_common_cpu_init(unsigned int cpu)
>> > >     void **inputarg, **outputarg;
>> > >     u64 msr_vp_index;
>> > >     gfp_t flags;
>> > > -   int pgcount = hv_root_partition ? 2 : 1;
>> > > +   const int pgcount = (hv_root_partition ||
>> > IS_ENABLED(CONFIG_HYPERV_VTL_MODE)) ? 2 : 1;
>> > >     void *mem;
>> > >     int ret;
>> > >
>> > > @@ -453,7 +453,7 @@ int hv_common_cpu_init(unsigned int cpu)
>> > >             if (!mem)
>> > >                     return -ENOMEM;
>> > >
>> > > -           if (hv_root_partition) {
>> > > +           if (hv_root_partition || IS_ENABLED(CONFIG_HYPERV_VTL_MODE)) {
>> > >                     outputarg = (void **)this_cpu_ptr(hyperv_pcpu_output_arg);
>> > >                     *outputarg = (char *)mem + HV_HYP_PAGE_SIZE;
>> > >             }
>> > > --
>> > > 2.34.1
>> > >
>>