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

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Message-ID: <2875aa3f-0dc4-4e48-17ad-42c703e12063@loongson.cn>
Date:   Tue, 21 Feb 2023 18:18:40 +0800
From:   maobibo <maobibo@...ngson.cn>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Tianrui Zhao <zhaotianrui@...ngson.cn>
Cc:     Huacai Chen <chenhuacai@...nel.org>,
        WANG Xuerui <kernel@...0n.name>,
        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



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

> 
> Can you please also add comments to kvm_vector_entry explaining the processor state after a VZ exception entry (interrupts, paging, ...)?
Yeap, we will add more comments about these critical exception entry.

Regards
Bibo, Mao
> 
> Paolo