linux-kernel - Re: [PATCH v5 1/8] KVM: arm64: Introduce hyp_alloc_private_va

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Date:   Tue, 8 Mar 2022 12:21:40 -0800
From:   Stephen Boyd <swboyd@...omium.org>
To:     Kalesh Singh <kaleshsingh@...gle.com>
Cc:     will@...nel.org, maz@...nel.org, qperret@...gle.com,
        tabba@...gle.com, surenb@...gle.com, kernel-team@...roid.com,
        James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        Mark Brown <broonie@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Peter Collingbourne <pcc@...gle.com>,
        "Madhavan T. Venkataraman" <madvenka@...ux.microsoft.com>,
        Andrew Scull <ascull@...gle.com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 1/8] KVM: arm64: Introduce hyp_alloc_private_va_range()

Quoting Kalesh Singh (2022-03-07 10:48:59)
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index bc2aba953299..ccb2847ee2f4 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -457,22 +457,17 @@ int create_hyp_mappings(void *from, void *to, enum kvm_pgtable_prot prot)
>         return 0;
>  }
>
> -static int __create_hyp_private_mapping(phys_addr_t phys_addr, size_t size,
> -                                       unsigned long *haddr,
> -                                       enum kvm_pgtable_prot prot)
> +
> +/**
> + * hyp_alloc_private_va_range - Allocates a private VA range.
> + * @size:      The size of the VA range to reserve.
> + *
> + * The private VA range is allocated below io_map_base and
> + * aligned based on the order of @size.

Add what it returns?

Return: Start address of allocated VA range or some error value... (I don't
understand this part).

It may also be a good idea to write out what VA is in the description:

The private virtual address (VA) range is allocated below io_map_base

> + */
> +unsigned long hyp_alloc_private_va_range(size_t size)
>  {
>         unsigned long base;
> -       int ret = 0;
> -
> -       if (!kvm_host_owns_hyp_mappings()) {
> -               base = kvm_call_hyp_nvhe(__pkvm_create_private_mapping,
> -                                        phys_addr, size, prot);
> -               if (IS_ERR_OR_NULL((void *)base))
> -                       return PTR_ERR((void *)base);
> -               *haddr = base;
> -
> -               return 0;
> -       }
>
>         mutex_lock(&kvm_hyp_pgd_mutex);
>
> @@ -484,29 +479,53 @@ static int __create_hyp_private_mapping(phys_addr_t phys_addr, size_t size,
>          *
>          * The allocated size is always a multiple of PAGE_SIZE.
>          */
> -       size = PAGE_ALIGN(size + offset_in_page(phys_addr));
> -       base = io_map_base - size;
> +       base = io_map_base - PAGE_ALIGN(size);
> +
> +       /* Align the allocation based on the order of its size */
> +       base = ALIGN_DOWN(base, PAGE_SIZE << get_order(size));
>
>         /*
>          * Verify that BIT(VA_BITS - 1) hasn't been flipped by
>          * allocating the new area, as it would indicate we've
>          * overflowed the idmap/IO address range.
>          */
> -       if ((base ^ io_map_base) & BIT(VA_BITS - 1))
> -               ret = -ENOMEM;
> +       if (!base || (base ^ io_map_base) & BIT(VA_BITS - 1))
> +               base = (unsigned long)ERR_PTR(-ENOMEM);

It looks odd to use an error pointer casted to unsigned long to return
from an address allocation function. Why not pass a pointer for base
like the function was written before and return an int from this
function with 0 for success and negative error value? Otherwise some
sort of define should made like DMA_MAPPING_ERROR and that can be used
to indicate to the caller that the allocation failed, or a simple zero
may work?

>         else
>                 io_map_base = base;
>
>         mutex_unlock(&kvm_hyp_pgd_mutex);
>
> -       if (ret)
> -               goto out;
> +       return base;
> +}
> +
> +static int __create_hyp_private_mapping(phys_addr_t phys_addr, size_t size,
> +                                       unsigned long *haddr,
> +                                       enum kvm_pgtable_prot prot)
> +{
> +       unsigned long addr;
> +       int ret = 0;
> +
> +       if (!kvm_host_owns_hyp_mappings()) {
> +               addr = kvm_call_hyp_nvhe(__pkvm_create_private_mapping,
> +                                        phys_addr, size, prot);
> +               if (IS_ERR((void *)addr))

IS_ERR_VALUE()?

> +                       return PTR_ERR((void *)addr);
> +               *haddr = addr;
> +
> +               return 0;
> +       }
> +
> +       size += offset_in_page(phys_addr);
> +       addr = hyp_alloc_private_va_range(size);
> +       if (IS_ERR((void *)addr))

IS_ERR_VALUE()?

> +               return PTR_ERR((void *)addr);
>
> -       ret = __create_hyp_mappings(base, size, phys_addr, prot);
> +       ret = __create_hyp_mappings(addr, size, phys_addr, prot);
>         if (ret)
>                 goto out;
>
> -       *haddr = base + offset_in_page(phys_addr);
> +       *haddr = addr + offset_in_page(phys_addr);
>  out:
>         return ret;

Would be simpler to remove the goto, or return early.

	if (!ret)
		*haddr = addr + offset_in_page(phys_addr);

	return ret;

>  }