linux-kernel - Re: [PATCH] KVM: x86/xen: Use hva_t for holding hypercall page address

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Message-ID: <303f907814d75e2f3bdec01564be745d312356d1.camel@amazon.co.uk>
Date:   Mon, 8 Feb 2021 20:20:44 +0000
From:   "Woodhouse, David" <dwmw@...zon.co.uk>
To:     "seanjc@...gle.com" <seanjc@...gle.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>
CC:     "jmattson@...gle.com" <jmattson@...gle.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "vkuznets@...hat.com" <vkuznets@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "joao.m.martins@...cle.com" <joao.m.martins@...cle.com>,
        "wanpengli@...cent.com" <wanpengli@...cent.com>
Subject: Re: [PATCH] KVM: x86/xen: Use hva_t for holding hypercall page address

On Mon, 2021-02-08 at 12:15 -0800, Sean Christopherson wrote:
> Use hva_t, a.k.a. unsigned long, for the local variable that holds the
> hypercall page address.  On 32-bit KVM, gcc complains about using a u64
> due to the implicit cast from a 64-bit value to a 32-bit pointer.
> 
>   arch/x86/kvm/xen.c: In function ‘kvm_xen_write_hypercall_page’:
>   arch/x86/kvm/xen.c:300:22: error: cast to pointer from integer of
>                              different size [-Werror=int-to-pointer-cast]
>   300 |   page = memdup_user((u8 __user *)blob_addr, PAGE_SIZE);

Thanks.

Acked-by: David Woodhouse <dwmw@...zon.co.uk>

> Cc: Joao Martins <joao.m.martins@...cle.com>
> Cc: David Woodhouse <dwmw@...zon.co.uk>
> Fixes: 23200b7a30de ("KVM: x86/xen: intercept xen hypercalls if enabled")
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/xen.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
> index 2cee0376455c..deda1ba8c18a 100644
> --- a/arch/x86/kvm/xen.c
> +++ b/arch/x86/kvm/xen.c
> @@ -286,8 +286,12 @@ int kvm_xen_write_hypercall_page(struct kvm_vcpu *vcpu, u64 data)
>                                 return 1;
>                 }
>         } else {
> -               u64 blob_addr = lm ? kvm->arch.xen_hvm_config.blob_addr_64
> -                                  : kvm->arch.xen_hvm_config.blob_addr_32;
> +               /*
> +                * Note, truncation is a non-issue as 'lm' is guaranteed to be
> +                * false for a 32-bit kernel, i.e. when hva_t is only 4 bytes.
> +                */
> +               hva_t blob_addr = lm ? kvm->arch.xen_hvm_config.blob_addr_64
> +                                    : kvm->arch.xen_hvm_config.blob_addr_32;
>                 u8 blob_size = lm ? kvm->arch.xen_hvm_config.blob_size_64
>                                   : kvm->arch.xen_hvm_config.blob_size_32;
>                 u8 *page;
> --
> 2.30.0.478.g8a0d178c01-goog
> 




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