linux-kernel - Re: [RFC PATCH 1/2] arm64: kvm: Save/restore MTE registers

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Message-ID: <313f5656-b306-72bb-5804-40d20a2cba1e@arm.com>
Date:   Thu, 18 Jun 2020 11:43:56 +0100
From:   Steven Price <steven.price@....com>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Marc Zyngier <maz@...nel.org>, Will Deacon <will@...nel.org>,
        James Morse <James.Morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki Poulose <Suzuki.Poulose@....com>,
        "kvmarm@...ts.cs.columbia.edu" <kvmarm@...ts.cs.columbia.edu>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Dave P Martin <Dave.Martin@....com>,
        Mark Rutland <Mark.Rutland@....com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC PATCH 1/2] arm64: kvm: Save/restore MTE registers

On 17/06/2020 15:05, Catalin Marinas wrote:
> On Wed, Jun 17, 2020 at 01:38:43PM +0100, Steven Price wrote:
>> diff --git a/arch/arm64/kvm/hyp/sysreg-sr.c b/arch/arm64/kvm/hyp/sysreg-sr.c
>> index 75b1925763f1..6ecee1528566 100644
>> --- a/arch/arm64/kvm/hyp/sysreg-sr.c
>> +++ b/arch/arm64/kvm/hyp/sysreg-sr.c
>> @@ -26,6 +26,12 @@
>>   static void __hyp_text __sysreg_save_common_state(struct kvm_cpu_context *ctxt)
>>   {
>>   	ctxt->sys_regs[MDSCR_EL1]	= read_sysreg(mdscr_el1);
>> +	if (system_supports_mte()) {
>> +		ctxt->sys_regs[RGSR_EL1] = read_sysreg_s(SYS_RGSR_EL1);
>> +		ctxt->sys_regs[GCR_EL1] = read_sysreg_s(SYS_GCR_EL1);
>> +		ctxt->sys_regs[TFSRE0_EL1] = read_sysreg_s(SYS_TFSRE0_EL1);
>> +		ctxt->sys_regs[TFSR_EL1] = read_sysreg_s(SYS_TFSR_EL1);
>> +	}
> 
> TFSR_EL1 is not a common register as we have the TFSR_EL2 as well. So
> you'd have to access it as read_sysreg_el1(SYS_TFSR) so that, in the VHE
> case, it generates TFSR_EL12, otherwise you just save the host register.

Ah, thanks for pointing that out - I'd got myself confused with the 
whole VHE _EL12 registers. I'd managed to miss that TFSR is banked.

> Also, since TFSR*_EL1 can be set asynchronously, I think we need to set
> the SCTLR_EL2.ITFSB bit so that the register update is synchronised on
> entry to EL2. With VHE we get this automatically as part of
> SCTLR_EL1_SET but it turns out that we have another SCTLR_ELx_FLAGS
> macro for the non-VHE case (why not calling this SCTLR_EL2_* I have no
> idea).

I hadn't noticed that there was a different set for the non-VHE case 
which was missing ITFSB - I'll update that.

Thanks,

Steve

>>   	/*
>>   	 * The host arm64 Linux uses sp_el0 to point to 'current' and it must
>> @@ -99,6 +105,12 @@ NOKPROBE_SYMBOL(sysreg_save_guest_state_vhe);
>>   static void __hyp_text __sysreg_restore_common_state(struct kvm_cpu_context *ctxt)
>>   {
>>   	write_sysreg(ctxt->sys_regs[MDSCR_EL1],	  mdscr_el1);
>> +	if (system_supports_mte()) {
>> +		write_sysreg_s(ctxt->sys_regs[RGSR_EL1], SYS_RGSR_EL1);
>> +		write_sysreg_s(ctxt->sys_regs[GCR_EL1], SYS_GCR_EL1);
>> +		write_sysreg_s(ctxt->sys_regs[TFSRE0_EL1], SYS_TFSRE0_EL1);
>> +		write_sysreg_s(ctxt->sys_regs[TFSR_EL1], SYS_TFSR_EL1);
>> +	}
> 
> Similarly here, you override the TFSR_EL2 with VHE enabled.
>