linux-kernel - Re: [PATCH v3 04/10] KVM: Implement kvm_put

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Message-ID: <20190822152854.GE25467@linux.intel.com>
Date:   Thu, 22 Aug 2019 08:28:54 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Steven Price <steven.price@....com>
Cc:     Marc Zyngier <maz@...nel.org>, Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        Catalin Marinas <catalin.marinas@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Russell King <linux@...linux.org.uk>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Pouloze <suzuki.poulose@....com>,
        Mark Rutland <mark.rutland@....com>, kvm@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 04/10] KVM: Implement kvm_put_guest()

On Wed, Aug 21, 2019 at 04:36:50PM +0100, Steven Price wrote:
> kvm_put_guest() is analogous to put_user() - it writes a single value to
> the guest physical address. The implementation is built upon put_user()
> and so it has the same single copy atomic properties.

What you mean by "single copy atomic"?  I.e. what guarantees does
put_user() provide that __copy_to_user() does not?

> 
> Signed-off-by: Steven Price <steven.price@....com>
> ---
>  include/linux/kvm_host.h | 24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)
> 
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index fcb46b3374c6..e154a1897e20 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -746,6 +746,30 @@ int kvm_write_guest_offset_cached(struct kvm *kvm, struct gfn_to_hva_cache *ghc,
>  				  unsigned long len);
>  int kvm_gfn_to_hva_cache_init(struct kvm *kvm, struct gfn_to_hva_cache *ghc,
>  			      gpa_t gpa, unsigned long len);
> +
> +#define __kvm_put_guest(kvm, gfn, offset, value, type)			\
> +({									\
> +	unsigned long __addr = gfn_to_hva(kvm, gfn);			\
> +	type __user *__uaddr = (type __user *)(__addr + offset);	\
> +	int __ret = 0;							\
> +									\
> +	if (kvm_is_error_hva(__addr))					\
> +		__ret = -EFAULT;					\
> +	else								\
> +		__ret = put_user(value, __uaddr);			\
> +	if (!__ret)							\
> +		mark_page_dirty(kvm, gfn);				\
> +	__ret;								\
> +})
> +
> +#define kvm_put_guest(kvm, gpa, value, type)				\
> +({									\
> +	gpa_t __gpa = gpa;						\
> +	struct kvm *__kvm = kvm;					\
> +	__kvm_put_guest(__kvm, __gpa >> PAGE_SHIFT,			\
> +			offset_in_page(__gpa), (value), type);		\
> +})
> +
>  int kvm_clear_guest_page(struct kvm *kvm, gfn_t gfn, int offset, int len);
>  int kvm_clear_guest(struct kvm *kvm, gpa_t gpa, unsigned long len);
>  struct kvm_memory_slot *gfn_to_memslot(struct kvm *kvm, gfn_t gfn);
> -- 
> 2.20.1
>