linux-kernel - RE: [PATCH v5 3/5] hyperv: Enable the hypercall output page for the VTL mode

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Message-ID:
 <SN6PR02MB41573C71F0BD479FA24A30E2D4152@SN6PR02MB4157.namprd02.prod.outlook.com>
Date: Fri, 3 Jan 2025 22:08:05 +0000
From: Michael Kelley <mhklinux@...look.com>
To: Stanislav Kinsburskii <skinsburskii@...ux.microsoft.com>, Roman Kisel
	<romank@...ux.microsoft.com>
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Subject: RE: [PATCH v5 3/5] hyperv: Enable the hypercall output page for the
 VTL mode

From: Stanislav Kinsburskii <skinsburskii@...ux.microsoft.com> 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@...ux.microsoft.com>
> > ---
> >  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
> >