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To:     Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org
Cc:     Stefano Stabellini <sstabellini@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>
References: <20210422151007.2205-1-jgross@suse.com>
 <20210422151007.2205-2-jgross@suse.com>
 <4282bb6f-1d19-4d00-d468-f5d4c7fb0f90@oracle.com>
From:   Juergen Gross <jgross@suse.com>
Subject: Re: [PATCH 1/3] xen: check required Xen features
Message-ID: <d53d6b83-2730-1ab6-7dba-236e86e247b3@suse.com>
Date:   Mon, 10 May 2021 15:21:51 +0200
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From: Juergen Gross <jgross@suse.com>
To: Boris Ostrovsky <boris.ostrovsky@oracle.com>,
 xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org
Cc: Stefano Stabellini <sstabellini@kernel.org>,
 Peter Zijlstra <peterz@infradead.org>
Message-ID: <d53d6b83-2730-1ab6-7dba-236e86e247b3@suse.com>
Subject: Re: [PATCH 1/3] xen: check required Xen features
References: <20210422151007.2205-1-jgross@suse.com>
 <20210422151007.2205-2-jgross@suse.com>
 <4282bb6f-1d19-4d00-d468-f5d4c7fb0f90@oracle.com>
In-Reply-To: <4282bb6f-1d19-4d00-d468-f5d4c7fb0f90@oracle.com>

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On 10.05.21 14:11, Boris Ostrovsky wrote:
>=20
> On 4/22/21 11:10 AM, Juergen Gross wrote:
>>  =20
>> +/*
>> + * Linux kernel expects at least Xen 4.0.
>> + *
>> + * Assume some features to be available for that reason (depending on=
 guest
>> + * mode, of course).
>> + */
>> +#define chk_feature(f) {						\
>> +		if (!xen_feature(f))					\
>> +			pr_err("Xen: feature %s not available!\n", #f);	\
>> +	}
>=20
>=20
> With your changes in the subsequent patches, are we still going to func=
tion properly without those features? (i.e. maybe we should just panic)

Depends on the use case.

XENFEAT_gnttab_map_avail_bits is relevant for driver domains using
user space backends only. In case it is not available "interesting"
things might happen.

XENFEAT_mmu_pt_update_preserve_ad not being present would result in
a subsequent mmu-update function using that feature returning -ENOSYS,
so this wouldn't be unrecognized.

So panic() might be a good idea in case the features are not available.

> (Also, chk_required_features() perhaps?)

Fine with me.


Juergen

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