linux-kernel - Re: [PATCH] KVM: s390: vsie: fix crycb virtual vs physical usage

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Date:   Mon, 5 Sep 2022 18:32:29 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Pierre Morel <pmorel@...ux.ibm.com>, kvm@...r.kernel.org
Cc:     linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        borntraeger@...ibm.com, frankja@...ux.ibm.com, cohuck@...hat.com,
        thuth@...hat.com, imbrenda@...ux.ibm.com, hca@...ux.ibm.com,
        gor@...ux.ibm.com, svens@...ux.ibm.com
Subject: Re: [PATCH] KVM: s390: vsie: fix crycb virtual vs physical usage

On 05.09.22 10:41, Pierre Morel wrote:
> Prepare VSIE for architectural changes where lowmem kernel real and
> kernel virtual address are different.

Bear with me, it used to be

	crycb = (struct kvm_s390_crypto_cb *) (unsigned long)crycb_o;
	apcb_o = (unsigned long) &crycb->apcb0;

and now it's

	apcb_o = crycb_o + offsetof(struct kvm_s390_crypto_cb, apcb0);


So the real issue seems to be

	crycb = (struct kvm_s390_crypto_cb *) (unsigned long)crycb_o;

because crycb_o actually is a guest address and not a host address.


But now I'm confused, because I would have thought that the result 
produced by both code would be identical (I completely agree that the 
new variant is better).

How does this interact with "lowmem kernel real and kernel virtual 
address are different." -- I would have thought that &crycb->apcb0 
doesn't actually access any memory and only performs arithmetical 
operations?

> 
> When we get the original crycb from the guest crycb we can use the
> phys_to_virt transformation, which will use the host transformations,
> but we must use an offset to calculate the guest real address apcb
> and give it to read_guest_real().

Can you elaborate where phys_to_virt() comes into play?

If this is an actual fix (as indicated in the patch subject), should 
this carry a

	Fixes: 56019f9aca22 ("KVM: s390: vsie: Allow CRYCB FORMAT-2")

-- 
Thanks,

David / dhildenb