linux-kernel - Re: Question on get random long worse in VM than on host

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Message-ID: <7530d60c-7419-fa86-57c1-9f52dc479598@huawei.com>
Date: Fri, 6 Sep 2024 11:42:24 +0800
From: Tangnianyao <tangnianyao@...wei.com>
To: Marc Zyngier <maz@...nel.org>
CC: Ard Biesheuvel <ardb@...nel.org>, Will Deacon <will@...nel.org>,
	<oliver.upton@...ux.dev>, <linux-arm-kernel@...ts.infradead.org>,
	<linux-kernel@...r.kernel.org>, <kvmarm@...ts.linux.dev>, "guoyang (C)"
	<guoyang2@...wei.com>
Subject: Re: Question on get random long worse in VM than on host



On 9/5/2024 16:17, Marc Zyngier wrote:
> [...]
>> Process is different between host and guest in arch/arm64, arch_get_random_seed_longs.
>> (1) In host , smccc_trng_available is false, it get randomness from RNDRRS.
>>
>> (2) In guest, smccc_trng_available is true, because kvm emulate it. Guest use smccc trng
>> and hvc, and trap to host kvm. Then in host call stack:
>> kvm_smccc_call_handler
>> kvm_trng_call
>> kvm_trng_do_rnd
>> get_random_long
>> ...
>> arch_get_random_seed_longs
>>
>> host get randomness as (1) and return random u64 to guest.
>> So the randomness guest finally get is from RNDRRS too.
>> Can we let guest get randomness directly from RNDRRS, not using hvc first?
>> The process for guest like (1):
>> 1) kvm not emulated smccc trng for guest
>> 2) guest check smccc trng, and get smccc_trng_available=false
>> 3) guest get randomness from RNDRRS
> I think I gave you the answer to this in my first reply [1].
>
> 	M.
>
> [1] https://lore.kernel.org/all/86zfotuoio.wl-maz@kernel.org/
>

Thank you Marc, I've missed this email.
Userspace should control  whether expose TRNG to guest.

Thanks.
Nianyao