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

[Jon Hunter]

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@xxxxxxxxxx> 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@xxxxxxxxxx>
> > ---
> >   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

--
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