linux-kernel - Re: [PATCH 2/2] KVM: selftests: Test forced instruction emulation in dirty log test (x86 only)

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Message-ID: <Zc-FXbxEfPNddiiL@linux.dev>
Date: Fri, 16 Feb 2024 07:55:09 -0800
From: Oliver Upton <oliver.upton@...ux.dev>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, David Matlack <dmatlack@...gle.com>,
	Pasha Tatashin <tatashin@...gle.com>,
	Michael Krebs <mkrebs@...gle.com>
Subject: Re: [PATCH 2/2] KVM: selftests: Test forced instruction emulation in
 dirty log test (x86 only)

On Thu, Feb 15, 2024 at 04:26:02PM -0800, Sean Christopherson wrote:
> On Thu, Feb 15, 2024, Oliver Upton wrote:
> > On Thu, Feb 15, 2024 at 01:33:48PM -0800, Sean Christopherson wrote:
> > 
> > [...]
> > 
> > > +/* TODO: Expand this madness to also support u8, u16, and u32 operands. */
> > > +#define vcpu_arch_put_guest(mem, val, rand) 						\
> > > +do {											\
> > > +	if (!is_forced_emulation_enabled || !(rand & 1)) {				\
> > > +		*mem = val;								\
> > > +	} else if (rand & 2) {								\
> > > +		__asm__ __volatile__(KVM_FEP "movq %1, %0"				\
> > > +				     : "+m" (*mem)					\
> > > +				     : "r" (val) : "memory");				\
> > > +	} else {									\
> > > +		uint64_t __old = READ_ONCE(*mem);					\
> > > +											\
> > > +		__asm__ __volatile__(KVM_FEP LOCK_PREFIX "cmpxchgq %[new], %[ptr]"	\
> > > +				     : [ptr] "+m" (*mem), [old] "+a" (__old)		\
> > > +				     : [new]"r" (val) : "memory", "cc");		\
> > > +	}										\
> > > +} while (0)
> > > +
> > 
> > Last bit of bikeshedding then I'll go... Can you just use a C function
> > and #define it so you can still do ifdeffery to slam in a default
> > implementation?
> 
> Yes, but the macro shenanigans aren't to create a default, they're to set the
> stage for expanding to other sizes without having to do:
> 
>   vcpu_arch_put_guest{8,16,32,64}()
> 
> or if we like bytes instead of bits:
> 
>   vcpu_arch_put_guest{1,2,4,8}()
> 
> I'm not completely against that approach; it's not _that_ much copy+paste
> boilerplate, but it's enough that I think that macros would be a clear win,
> especially if we want to expand what instructions are used.

Oh, I see what you're after. Yeah, macro shenanigans are the only way
out then. Wasn't clear to me if the interface you wanted w/ the selftest
was a u64 write that you cracked into multiple writes behind the
scenes.

Thanks for entertaining my questions :)

-- 
Thanks,
Oliver