linux-kernel - Re: [PATCH 0/5] kvm: fix latent guest entry/exit bugs

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Message-ID: <20220118120154.GA17938@C02TD0UTHF1T.local>
Date:   Tue, 18 Jan 2022 12:02:06 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Christian Borntraeger <borntraeger@...ux.ibm.com>,
        linux-kernel@...r.kernel.org, aleksandar.qemu.devel@...il.com,
        alexandru.elisei@....com, anup.patel@....com,
        aou@...s.berkeley.edu, atish.patra@....com,
        benh@...nel.crashing.org, bp@...en8.de, catalin.marinas@....com,
        chenhuacai@...nel.org, dave.hansen@...ux.intel.com,
        david@...hat.com, frankja@...ux.ibm.com, frederic@...nel.org,
        gor@...ux.ibm.com, hca@...ux.ibm.com, imbrenda@...ux.ibm.com,
        james.morse@....com, jmattson@...gle.com, joro@...tes.org,
        kvm@...r.kernel.org, maz@...nel.org, mingo@...hat.com,
        mpe@...erman.id.au, nsaenzju@...hat.com, palmer@...belt.com,
        paulmck@...nel.org, paulus@...ba.org, paul.walmsley@...ive.com,
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        tsbogend@...ha.franken.de, vkuznets@...hat.com,
        wanpengli@...cent.com, will@...nel.org
Subject: Re: [PATCH 0/5] kvm: fix latent guest entry/exit bugs

On Mon, Jan 17, 2022 at 06:45:36PM +0100, Paolo Bonzini wrote:
> On 1/14/22 16:19, Mark Rutland wrote:
> > I also think there is another issue here. When an IRQ is taken from SIE, will
> > user_mode(regs) always be false, or could it be true if the guest userspace is
> > running? If it can be true I think tha context tracking checks can complain,
> > and it*might*  be possible to trigger a panic().
> 
> I think that it would be false, because the guest PSW is in the SIE block
> and switched on SIE entry and exit, but I might be incorrect.

Ah; that's the crux of my confusion: I had thought the guest PSW would
be placed in the regular lowcore *_old_psw slots. From looking at the
entry asm it looks like the host PSW (around the invocation of SIE) is
stored there, since that's what the OUTSIDE + SIEEXIT handling is
checking for.

Assuming that's correct, I agree this problem doesn't exist, and there's
only the common RCU/tracing/lockdep management to fix.

Sorry for the noise, and thanks for the pointer!

Thanks,
Mark.