linux-kernel - Re: [PATCH v2 3/3] KVM: s390: gaccess: Cleanup access to guest frames

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Message-ID: <4ac7c459-8e13-087a-f98d-9f3e0e6d8ee6@redhat.com>
Date:   Thu, 28 Oct 2021 16:25:07 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Janis Schoetterl-Glausch <scgl@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>
Cc:     Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        kvm@...r.kernel.org, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/3] KVM: s390: gaccess: Cleanup access to guest frames

On 28.10.21 15:55, Janis Schoetterl-Glausch wrote:
> Introduce a helper function for guest frame access.

"guest page access"

But I do wonder if you actually want to call it

"access_guest_abs"

and say "guest absolute access" instead here.

Because we're dealing with absolute addresses and the fact that we are
accessing it page-wise is just because we have to perform a page-wise
translation in the callers (either virtual->absolute or real->absolute).

Theoretically, if you know you're across X pages but they are contiguous
in absolute address space, nothing speaks against using that function
directly across X pages with a single call.

> 
> Signed-off-by: Janis Schoetterl-Glausch <scgl@...ux.ibm.com>
> ---
>  arch/s390/kvm/gaccess.c | 24 ++++++++++++++++--------
>  1 file changed, 16 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/s390/kvm/gaccess.c b/arch/s390/kvm/gaccess.c
> index f0848c37b003..9a633310b6fe 100644
> --- a/arch/s390/kvm/gaccess.c
> +++ b/arch/s390/kvm/gaccess.c
> @@ -866,6 +866,20 @@ static int guest_range_to_gpas(struct kvm_vcpu *vcpu, unsigned long ga, u8 ar,
>  	return 0;
>  }
>  
> +static int access_guest_page(struct kvm *kvm, enum gacc_mode mode, gpa_t gpa,
> +			      void *data, unsigned int len)
> +{
> +	const unsigned int offset = offset_in_page(gpa);
> +	const gfn_t gfn = gpa_to_gfn(gpa);
> +	int rc;
> +
> +	if (mode == GACC_STORE)
> +		rc = kvm_write_guest_page(kvm, gfn, data, offset, len);
> +	else
> +		rc = kvm_read_guest_page(kvm, gfn, data, offset, len);
> +	return rc;
> +}
> +
>  int access_guest(struct kvm_vcpu *vcpu, unsigned long ga, u8 ar, void *data,
>  		 unsigned long len, enum gacc_mode mode)
>  {
> @@ -896,10 +910,7 @@ int access_guest(struct kvm_vcpu *vcpu, unsigned long ga, u8 ar, void *data,
>  	rc = guest_range_to_gpas(vcpu, ga, ar, gpas, len, asce, mode);
>  	for (idx = 0; idx < nr_pages && !rc; idx++) {
>  		fragment_len = min(PAGE_SIZE - offset_in_page(gpas[idx]), len);
> -		if (mode == GACC_STORE)
> -			rc = kvm_write_guest(vcpu->kvm, gpas[idx], data, fragment_len);
> -		else
> -			rc = kvm_read_guest(vcpu->kvm, gpas[idx], data, fragment_len);
> +		rc = access_guest_page(vcpu->kvm, mode, gpas[idx], data, fragment_len);
>  		len -= fragment_len;
>  		data += fragment_len;
>  	}
> @@ -920,10 +931,7 @@ int access_guest_real(struct kvm_vcpu *vcpu, unsigned long gra,
>  	while (len && !rc) {
>  		gpa = kvm_s390_real_to_abs(vcpu, gra);
>  		fragment_len = min(PAGE_SIZE - offset_in_page(gpa), len);
> -		if (mode)
> -			rc = write_guest_abs(vcpu, gpa, data, fragment_len);
> -		else
> -			rc = read_guest_abs(vcpu, gpa, data, fragment_len);
> +		rc = access_guest_page(vcpu->kvm, mode, gpa, data, fragment_len);
>  		len -= fragment_len;
>  		gra += fragment_len;
>  		data += fragment_len;
> 


-- 
Thanks,

David / dhildenb