linux-kernel - Re: [PATCH v4 5/6] KVM: selftests: Make arm64's MMIO ucall multi-VM friendly

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Message-ID: <YwZEdzHtWEfCpr7B@google.com>
Date:   Wed, 24 Aug 2022 15:32:07 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Oliver Upton <oliver.upton@...ux.dev>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, Marc Zyngier <maz@...nel.org>,
        Anup Patel <anup@...infault.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Atish Patra <atishp@...shpatra.org>,
        David Hildenbrand <david@...hat.com>,
        Tom Rix <trix@...hat.com>, kvm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        kvm-riscv@...ts.infradead.org, linux-riscv@...ts.infradead.org,
        llvm@...ts.linux.dev, linux-kernel@...r.kernel.org,
        Colton Lewis <coltonlewis@...gle.com>,
        Peter Gonda <pgonda@...gle.com>,
        Andrew Jones <andrew.jones@...ux.dev>
Subject: Re: [PATCH v4 5/6] KVM: selftests: Make arm64's MMIO ucall multi-VM
 friendly

On Wed, Aug 24, 2022, Oliver Upton wrote:
> Hi Sean,
> 
> On Wed, Aug 24, 2022 at 03:21:14AM +0000, Sean Christopherson wrote:
> > +/*
> > + * Sync a global pointer to the guest that has a per-VM value, in which case
> > + * writes to the host copy of the "global" must be serialized (in case a test
> > + * is being truly crazy and spawning multiple VMs concurrently).
> > + */
> 
> Do we even care about writes to the host's copy of the global pointer?
> I don't see how the host pointer is used beyond serializing writes into
> a guest.
> 
> IOW, it looks as though we could skip the whole global illusion
> altogether and write straight into guest memory.

*sigh*

This exact thought crossed my mind when I first looked at this code, but somehow
I couldn't come up with the obvious solution of using a temporary on-stack variable
to hold the desired value.

Something like this should work.

#define write_guest_global(vm, g, val) ({			\
	typeof(g) *_p = addr_gva2hva(vm, (vm_vaddr_t)&(g));	\
	typeof(g) _val = val;					\
								\
	memcpy(_p, &(_val), sizeof(g));				\
})