Re: [Patch] memory: tegra: Skip SID override from Guest VM

[Jon Hunter <jonathanh@nvidia.com>]

Hi Marc,

On 06/02/2024 12:17, Marc Zyngier wrote:
> Hi Sumit,
> 
> On Tue, 06 Feb 2024 11:48:52 +0000,
> Sumit Gupta <sumitg@nvidia.com> wrote:
>>
>> MC SID register access is restricted for Guest VM.
>> So, skip the SID override programming from the Guest VM.
>>
>> Signed-off-by: Sumit Gupta <sumitg@nvidia.com>
>> ---
>>   drivers/memory/tegra/tegra186.c | 11 +++++++++++
>>   1 file changed, 11 insertions(+)
>>
>> diff --git a/drivers/memory/tegra/tegra186.c b/drivers/memory/tegra/tegra186.c
>> index 1b3183951bfe..df441896b69d 100644
>> --- a/drivers/memory/tegra/tegra186.c
>> +++ b/drivers/memory/tegra/tegra186.c
>> @@ -10,6 +10,7 @@
>>   #include <linux/of.h>
>>   #include <linux/of_platform.h>
>>   #include <linux/platform_device.h>
>> +#include <asm/virt.h>
>>   
>>   #include <soc/tegra/mc.h>
>>   
>> @@ -118,6 +119,11 @@ static int tegra186_mc_probe_device(struct tegra_mc *mc, struct device *dev)
>>   	unsigned int i, index = 0;
>>   	u32 sid;
>>   
>> +	if (!is_kernel_in_hyp_mode()) {
>> +		dev_dbg(mc->dev, "Register access not allowed\n");
>> +		return 0;
>> +	}
>> +
>>   	if (!tegra_dev_iommu_get_stream_id(dev, &sid))
>>   		return 0;
>>   
>> @@ -146,6 +152,11 @@ static int tegra186_mc_resume(struct tegra_mc *mc)
>>   #if IS_ENABLED(CONFIG_IOMMU_API)
>>   	unsigned int i;
>>   
>> +	if (!is_kernel_in_hyp_mode()) {
>> +		dev_dbg(mc->dev, "Register access not allowed\n");
>> +		return 0;
>> +	}
>> +
>>   	for (i = 0; i < mc->soc->num_clients; i++) {
>>   		const struct tegra_mc_client *client = &mc->soc->clients[i];
>>   
> 
> This doesn't look right. Multiple reasons:
> 
> - This helper really has nothing to do in a driver. This is
>    architectural stuff that is not intended for use outside of arch
>    core code.

We see a few other drivers using this ...

drivers/perf/arm_pmuv3.c
drivers/clocksource/arm_arch_timer.c
drivers/gpu/drm/amd/amdgpu/amdgpu_virt.h
drivers/hwtracing/coresight/coresight-etm4x-core.c
drivers/hwtracing/coresight/coresight-etm4x-core.c
drivers/irqchip/irq-apple-aic.c

We were looking for a way to determine if the OS is a guest OS or not. 
However, I can see that this is a ARM64 specific API and so probably the 
above are only compiled for ARM64. Interestingly, the AMD driver 
implements the following ...

static inline bool is_virtual_machine(void)
{
#if defined(CONFIG_X86)
         return boot_cpu_has(X86_FEATURE_HYPERVISOR);
#elif defined(CONFIG_ARM64)
         return !is_kernel_in_hyp_mode();
#else
         return false;
#endif
}

> - My own tegra186 HW doesn't have VHE, since it is ARMv8.0, and this
>    helper will always return 'false'. How could this result in
>    something that still works? Can I get a free CPU upgrade?

I thought this API just checks to see if we are in EL2?

> - If you assign this device to a VM and that the hypervisor doesn't
>    correctly virtualise it, then it is a different device and you
>    should simply advertise it something else. Or even better, fix your
>    hypervisor.

Sumit can add some more details on why we don't completely disable the 
device for guest OSs.

Jon

-- 
nvpublic