linux-kernel - Re: [PATCH v2 2/6] drivers/hv: Enable VTL mode for arm64

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Date: Wed, 15 May 2024 11:04:13 -0700
From: Roman Kisel <romank@...ux.microsoft.com>
To: Michael Kelley <mhklinux@...look.com>, "arnd@...db.de" <arnd@...db.de>,
 "bhelgaas@...gle.com" <bhelgaas@...gle.com>, "bp@...en8.de" <bp@...en8.de>,
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Cc: "ssengar@...rosoft.com" <ssengar@...rosoft.com>,
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 "vdso@...bites.dev" <vdso@...bites.dev>
Subject: Re: [PATCH v2 2/6] drivers/hv: Enable VTL mode for arm64



On 5/15/2024 6:37 AM, Michael Kelley wrote:
> From: Roman Kisel <romank@...ux.microsoft.com> Sent: Tuesday, May 14, 2024 3:44 PM
>>
>> Kconfig dependencies for arm64 guests on Hyper-V require that be ACPI enabled,
>> and limit VTL mode to x86/x64. To enable VTL mode on arm64 as well, update the
>> dependencies. Since VTL mode requires DeviceTree instead of ACPI, don't require
>> arm64 guests on Hyper-V to have ACPI.
>>
>> Signed-off-by: Roman Kisel <romank@...ux.microsoft.com>
>> ---
>>   drivers/hv/Kconfig | 6 +++---
>>   1 file changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/hv/Kconfig b/drivers/hv/Kconfig
>> index 862c47b191af..a5cd1365e248 100644
>> --- a/drivers/hv/Kconfig
>> +++ b/drivers/hv/Kconfig
>> @@ -5,7 +5,7 @@ menu "Microsoft Hyper-V guest support"
>>   config HYPERV
>>   	tristate "Microsoft Hyper-V client drivers"
>>   	depends on (X86 && X86_LOCAL_APIC && HYPERVISOR_GUEST) \
>> -		|| (ACPI && ARM64 && !CPU_BIG_ENDIAN)
>> +		|| (ARM64 && !CPU_BIG_ENDIAN)
>>   	select PARAVIRT
>>   	select X86_HV_CALLBACK_VECTOR if X86
>>   	select OF_EARLY_FLATTREE if OF
>> @@ -15,7 +15,7 @@ config HYPERV
>>
>>   config HYPERV_VTL_MODE
>>   	bool "Enable Linux to boot in VTL context"
>> -	depends on X86_64 && HYPERV
>> +	depends on HYPERV
>>   	depends on SMP
>>   	default n
>>   	help
> 
> These changes make it possible to build a normal VTL 0 Hyper-V
> guest (i.e., CONFIG_HYPERV_VTL_MODE=n) if CONFIG_ACPI is
> not set, which won't work.  While we can say "don't do that", it
> would be better if the Kconfig dependencies expressed that
> requirement.
> 
> A possible fix is to remove the "depends on HYPERV" from
> HYPERV_VTL_MODE.  Then for HYPERV, make
> the "depends on ACPI" be conditional on !HYPERV_VTL_MODE
> (for both ARM64 and X86).
> 
> I think we originally had "depends on HYPERV" in
> HYPERV_VTL_MODE because there was a VTL-related function
> in a non-Hyper-V code path, and we wanted to prevent that code
> from running in non-Hyper-V environments.  But in practice, that
> turned out not to work well because occasionally people would
> do an "all config" build where both CONFIG_HYPERV and
> CONFIG_HYPERV_VTL_MODE were set, and it would panic during
> boot in their non-Hyper-V environment.  Such people were not
> happy. :-(  So Saurabh made a relatively simple change (see commit
> 14058f72cf13e) that got the VTL code out of that non-Hyper-V code
> path.  With that change, it shouldn't matter if someone sets
> CONFIG_HYPERV_VTL_MODE=y in a build where
> CONFIG_HYPERV=n.
> 
> At least that's my theory. :-)  Someone would need to check
> it carefully.
I'll explore that, appreciate sharing the context!

> 
> Michael

-- 
Thank you,
Roman