linux-kernel - Re: [PATCH v14 4/6] KVM: s390: pv: avoid export before import if possible

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Date:   Tue, 4 Oct 2022 15:47:43 +0200
From:   Steffen Eiden <seiden@...ux.ibm.com>
To:     Claudio Imbrenda <imbrenda@...ux.ibm.com>, kvm@...r.kernel.org
Cc:     borntraeger@...ibm.com, frankja@...ux.ibm.com, thuth@...hat.com,
        david@...hat.com, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org, scgl@...ux.ibm.com, nrb@...ux.ibm.com
Subject: Re: [PATCH v14 4/6] KVM: s390: pv: avoid export before import if
 possible



On 9/30/22 16:01, Claudio Imbrenda wrote:
> If the appropriate UV feature bit is set, there is no need to perform
> an export before import.
> 
> The misc feature indicates, among other things, that importing a shared
> page from a different protected VM will automatically also transfer its
> ownership.
> 
> Signed-off-by: Claudio Imbrenda <imbrenda@...ux.ibm.com>
> Reviewed-by: Nico Boehr <nrb@...ux.ibm.com>
> Reviewed-by: Janosch Frank <frankja@...ux.ibm.com>
LGTM
Reviewed-by: Steffen Eiden <seiden@...ux.ibm.com>

> ---
>   arch/s390/kernel/uv.c | 7 +++++++
>   1 file changed, 7 insertions(+)
> 
> diff --git a/arch/s390/kernel/uv.c b/arch/s390/kernel/uv.c
> index f9810d2a267c..9f18a4af9c13 100644
> --- a/arch/s390/kernel/uv.c
> +++ b/arch/s390/kernel/uv.c
> @@ -255,6 +255,13 @@ static int make_secure_pte(pte_t *ptep, unsigned long addr,
>    */
>   static bool should_export_before_import(struct uv_cb_header *uvcb, struct mm_struct *mm)
>   {
> +	/*
> +	 * The misc feature indicates, among other things, that importing a
> +	 * shared page from a different protected VM will automatically also
> +	 * transfer its ownership.
> +	 */
> +	if (test_bit_inv(BIT_UV_FEAT_MISC, &uv_info.uv_feature_indications))
> +		return false;
>   	if (uvcb->cmd == UVC_CMD_UNPIN_PAGE_SHARED)
>   		return false;
>   	return atomic_read(&mm->context.protected_count) > 1;