bhyve: when accessing non-backed gpa, emulate hw

[kib]

Summary

which reads all bits one, and ignores writes.

PR:     286467

[markj]

usr.sbin/bhyve/mem.c

195

Maybe make this a global variable?

198

[kib]

Make fb_entry static. Reword the message.

[kib]

Drop now unused string.h

[markj]

One issue with this is that the code is MI, but the implemented behaviour is x86-specific -- I'm not sure what happens on arm64 when a non-existent GPA is accessed, maybe an serror is raised. Maybe @andrew could comment.

[kib]

usr.sbin/bhyve/mem.c

198

I reworded as you suggested. But note that the address always exists, it is the backing memory that could not.

[kib]

Restore original behavior on !x86.

[kib]

Eliminate arch ifdefs, move code into mem_x86.c, mem_md.c.

[kib]

Stray ifdef.

[kib]

Another stray change, sorry.

[andrew]

In D50116#1143835, @markj wrote:

One issue with this is that the code is MI, but the implemented behaviour is x86-specific -- I'm not sure what happens on arm64 when a non-existent GPA is accessed, maybe an serror is raised. Maybe @andrew could comment.

On arm64 I would expect we implement it as a synchronous external abort. We should have all the information needed & it lets the guest recover, or at least report the precise location of the fault.

[kib]

On armv8, generate simple-minded SError.

[andrew]

usr.sbin/bhyve/aarch64/mem_aarch64.c

48 ↗

(On Diff #155174)

I don't think this is correct as it looks like this should only be raised for an SError exception. The kernel can currently only raise synchronous exceptions. I think the logic should be:

spsr = get(VM_REG_GUEST_CPSR);
if ((spsr & PSR_M_MASK) == PSR_M_EL0t)
    esr = EXCP_DATA_ABORT_L << ESR_ELx_EC_SHIFT;
else
    esr = EXCP_DATA_ABORT << ESR_ELx_EC_SHIFT;
esr |= ESR_ELx_IL | ISS_DATA_DFSC_EXT;

[kib]

Add needed headers for mem_aarch64.c

[kib]

Implement andrew' suggestion.

[andrew]

I'm ok with the arm64 parts