linux-kernel - Re: [PATCH 1/2] KVM: s390: gaccess: Cleanup access to guest frames

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Date:   Wed, 18 Aug 2021 09:54:49 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Janis Schoetterl-Glausch <scgl@...ux.ibm.com>, kvm@...r.kernel.org,
        borntraeger@...ibm.com, frankja@...ux.ibm.com,
        imbrenda@...ux.ibm.com, Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>
Cc:     cohuck@...hat.com, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] KVM: s390: gaccess: Cleanup access to guest frames

On 16.08.21 17:07, Janis Schoetterl-Glausch wrote:
> Introduce a helper function for guest frame access.
> Rewrite the calculation of the gpa and the length of the segment
> to be more readable.
> 
> Signed-off-by: Janis Schoetterl-Glausch <scgl@...ux.ibm.com>
> ---
>   arch/s390/kvm/gaccess.c | 48 +++++++++++++++++++++++++----------------
>   1 file changed, 29 insertions(+), 19 deletions(-)
> 
> diff --git a/arch/s390/kvm/gaccess.c b/arch/s390/kvm/gaccess.c
> index b9f85b2dc053..df83de0843de 100644
> --- a/arch/s390/kvm/gaccess.c
> +++ b/arch/s390/kvm/gaccess.c
> @@ -827,11 +827,26 @@ static int guest_page_range(struct kvm_vcpu *vcpu, unsigned long ga, u8 ar,
>   	return 0;
>   }
>   
> +static int access_guest_frame(struct kvm *kvm, enum gacc_mode mode, gpa_t gpa,
> +			      void *data, unsigned int len)

I know, "frame" is a beautiful term for "page" -- can we just avoid 
using it because we're not using it anywhere else here? :)

What's wrong with "access_guest_page()" ?


> +{
> +	gfn_t gfn = gpa_to_gfn(gpa);
> +	unsigned int offset = offset_in_page(gpa);
> +	int rc;

You could turn both const. You might want to consider 
reverse-christmas-treeing this.

> +
> +	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);

Personally, I prefer passing in pfn + offset instead of a gpa. Also 
avoids having to convert back and forth.

> +	return rc;
> +}
> +
>   int access_guest(struct kvm_vcpu *vcpu, unsigned long ga, u8 ar, void *data,
>   		 unsigned long len, enum gacc_mode mode)
>   {
>   	psw_t *psw = &vcpu->arch.sie_block->gpsw;
> -	unsigned long _len, nr_pages, gpa, idx;
> +	unsigned long nr_pages, gpa, idx;
> +	unsigned int seg;

Dito, reverse christmas tree might be worth keeping.

>   	unsigned long pages_array[2];
>   	unsigned long *pages;
>   	int need_ipte_lock;
> @@ -855,15 +870,12 @@ int access_guest(struct kvm_vcpu *vcpu, unsigned long ga, u8 ar, void *data,
>   		ipte_lock(vcpu);
>   	rc = guest_page_range(vcpu, ga, ar, pages, nr_pages, asce, mode);
>   	for (idx = 0; idx < nr_pages && !rc; idx++) {
> -		gpa = *(pages + idx) + (ga & ~PAGE_MASK);
> -		_len = min(PAGE_SIZE - (gpa & ~PAGE_MASK), len);
> -		if (mode == GACC_STORE)
> -			rc = kvm_write_guest(vcpu->kvm, gpa, data, _len);
> -		else
> -			rc = kvm_read_guest(vcpu->kvm, gpa, data, _len);
> -		len -= _len;
> -		ga += _len;
> -		data += _len;
> +		gpa = pages[idx] + offset_in_page(ga);
> +		seg = min(PAGE_SIZE - offset_in_page(gpa), len);
> +		rc = access_guest_frame(vcpu->kvm, mode, gpa, data, seg);
> +		len -= seg;
> +		ga += seg;
> +		data += seg;
>   	}
>   	if (need_ipte_lock)
>   		ipte_unlock(vcpu);
> @@ -875,19 +887,17 @@ int access_guest(struct kvm_vcpu *vcpu, unsigned long ga, u8 ar, void *data,
>   int access_guest_real(struct kvm_vcpu *vcpu, unsigned long gra,
>   		      void *data, unsigned long len, enum gacc_mode mode)
>   {
> -	unsigned long _len, gpa;
> +	unsigned long gpa;
> +	unsigned int seg;
>   	int rc = 0;
>   
>   	while (len && !rc) {
>   		gpa = kvm_s390_real_to_abs(vcpu, gra);
> -		_len = min(PAGE_SIZE - (gpa & ~PAGE_MASK), len);
> -		if (mode)
> -			rc = write_guest_abs(vcpu, gpa, data, _len);
> -		else
> -			rc = read_guest_abs(vcpu, gpa, data, _len);
> -		len -= _len;
> -		gra += _len;
> -		data += _len;
> +		seg = min(PAGE_SIZE - offset_in_page(gpa), len);

What does "seg" mean? I certainly know when "len" means -- which is also 
what the function eats.

> +		rc = access_guest_frame(vcpu->kvm, mode, gpa, data, seg);
> +		len -= seg;
> +		gra += seg;
> +		data += seg;
>   	}
>   	return rc;
>   }
> 


-- 
Thanks,

David / dhildenb