linux-kernel - Re: [PATCH v2 02/29] LoongArch: KVM: Implement kvm module related interface

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Message-ID: <35d54051-3876-4fb7-d1c8-d1c605420962@xen0n.name>
Date:   Tue, 21 Feb 2023 18:37:27 +0800
From:   WANG Xuerui <kernel@...0n.name>
To:     maobibo <maobibo@...ngson.cn>, Paolo Bonzini <pbonzini@...hat.com>,
        Tianrui Zhao <zhaotianrui@...ngson.cn>
Cc:     Huacai Chen <chenhuacai@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        loongarch@...ts.linux.dev, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
        Mark Brown <broonie@...nel.org>,
        Alex Deucher <alexander.deucher@....com>,
        Oliver Upton <oliver.upton@...ux.dev>
Subject: Re: [PATCH v2 02/29] LoongArch: KVM: Implement kvm module related
 interface

On 2023/2/21 18:18, maobibo wrote:
> 
> 
> 在 2023/2/21 16:14, Paolo Bonzini 写道:
>> On 2/21/23 07:59, maobibo wrote:
>>>> Also, why does the world switch code need a copy?
>>> There will be problem in world switch code if there is page fault reenter,
>>> since pgd register is shared between root kernel and kvm hypervisor.
>>> World switch entry need be unmapped area, cannot be tlb mapped area.
>>
>> So if I understand correctly the processor is in direct address translation mode until the "csrwr t0, LOONGARCH_CSR_CRMD" instruction. Where does it leave paged mode?
> The processor still in paged mode during world switch context. For example
> when vm exits from guest mode to root mode, it executes world switch code
> from kvm_vector_entry, PC register points to HVA address, however vmid from
> LOONGARCH_CSR_GTLBC is not clear to root mode. If there is page fault
> exception, hardware treats it exception from GPA-->HPA rather than that
> from HVA --> HPA, since vmid info in CSR_GTLBC is not zero.
> 
> In page mode, there are two kinds of address: unmapped address and
> tlb mapped address. For unmapped address there is only cachable/uncachable
> attribution, but not RWX attr; and there is no tlb handling for it.
> For simplicity,  unmapped address can be treated as window filtered address.
> 
> It will be fully root mode only after this piece of code is executed
> during world switch context; vmid is zero and PC points to HVA.
>          ori     t0, zero, CSR_GSTAT_PVM
>          csrxchg zero, t0, LOONGARCH_CSR_GSTAT
>          /* Clear GTLBC.TGID field */
>          csrrd   t0, LOONGARCH_CSR_GTLBC
>          bstrins.w       t0, zero, CSR_GTLBC_TGID_SHIFT_END, CSR_GTLBC_TGID_SHIFT
>          csrwr   t0, LOONGARCH_CSR_GTLBC

AFAIK all of these is probably coming from Volume 3 of LoongArch ISA 
Manual, which is unfortunately not publicly available at the moment. For 
sake of meaningful reviews, when can we expect to get our hands on the 
manuals?

-- 
WANG "xen0n" Xuerui

Linux/LoongArch mailing list: https://lore.kernel.org/loongarch/