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

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Date: Tue, 06 Feb 2024 12:17:52 +0000
From: Marc Zyngier <maz@...nel.org>
To: Sumit Gupta <sumitg@...dia.com>
Cc: <treding@...dia.com>,
	<krzysztof.kozlowski@...aro.org>,
	<jonathanh@...dia.com>,
	<mark.rutland@....com>,
	<linux-kernel@...r.kernel.org>,
	<linux-tegra@...r.kernel.org>,
	<amhetre@...dia.com>,
	<bbasu@...dia.com>
Subject: Re: [Patch] memory: tegra: Skip SID override from Guest VM

Hi Sumit,

On Tue, 06 Feb 2024 11:48:52 +0000,
Sumit Gupta <sumitg@...dia.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@...dia.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.

- 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?

- 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.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.